Question: Broad Top is a plateau located in south-central Pennsylvania bounded to the east by what long, steep, narrow mountain ridge in the Ridge-and-Valley (or Allegheny Mountains) physiographic province of the Appalachian Mountains?

Context paragraphs:
List of mountains of the Alleghenies: This list of ridges and summits of the Allegheny Mountains identifies geographic elevations for about 500 miles (800 km) from north central Pennsylvania, through eastern West Virginia and western Maryland, to western Virginia in the USA.  The range of the Allegheny Mountains is part of the Ridge-and-Valley Appalachians, a physiographic region of the much larger Appalachian Mountain Range.

Broad Top: Broad Top is a plateau located in south-central Pennsylvania.  It extends into Huntingdon County to the north, Fulton County to the southeast, and Bedford County to the southwest.  It is bounded to the west by Saxton Mountain and Terrace Mountain, and to the east by Sideling Hill.  In Bedford County, Harbor Mountain forms the southern boundary.  Trough Creek Valley lies between the mountains.

Wallkill Valley: The Wallkill Valley is a broad valley extending through southeastern New York and northwestern New Jersey.  It is composed of rolling hills, plains, and swamps (including the Black Dirt Region) surrounding the Wallkill River.  The valley is a subdivision of the larger Hudson Valley, bound to the west by the Shawangunk Ridge/Kittatinny Mountains and to the east by the Marlboro Mountains and New York–New Jersey Highlands.  The northern Wallkill Valley is sometimes associated with the greater Catskills region, although it is geographically separated from the Catskill Mountains by the Shawangunk Ridge and Rondout Valley.  In a broader sense, the Wallkill Valley is part of the Ridge-and-Valley physiographic province of the Appalachian Mountains, while the Catskills further to the north and west are part of the Appalachian Plateau.

Sideling Hill: Sideling Hill (also Side Long Hill) is a long, steep, narrow mountain ridge in the Ridge-and-Valley (or Allegheny Mountains) physiographic province of the Appalachian Mountains, located in Washington County in western Maryland and adjacent West Virginia and Pennsylvania, USA.  The highest point on the ridge is Fisher Point, at 2310 ft in Fulton County, Pennsylvania.

Blue Ridge Mountains: The Blue Ridge Mountains are a physiographic province of the larger Appalachian Mountains range.  This province consists of northern and southern physiographic regions, which divide near the Roanoke River gap.  The mountain range is located in the eastern United States, starting at its southernmost portion in Georgia, then ending northward in Pennsylvania.  To the west of the Blue Ridge, between it and the bulk of the Appalachians, lies the Great Appalachian Valley, bordered on the west by the Ridge and Valley province of the Appalachian range.

Allegheny Front: The Allegheny Front is the major southeast- or east-facing escarpment in the Allegheny Mountains in southern Pennsylvania, western Maryland, and eastern West Virginia, USA.  The Allegheny Front forms the boundary between the Ridge-and-Valley Appalachians to its east and the Appalachian Plateau (locally called the Allegheny Plateau) to its west.  The Front is closely associated with the Appalachian Mountains' Eastern Continental Divide, which in this area divides the waters of the Ohio/Mississippi river system, flowing to the Gulf of Mexico, from rivers flowing into Chesapeake Bay and from there into the Atlantic Ocean.

Rays Hill: Rays Hill is a mountain ridge in Pennsylvania's Ridge and Valley Appalachians region.  It is bordered to the east by Sideling Hill.  About halfway along its run, the west side of Rays Hill ties into Broad Top Mountain, a large plateau.  On its west it is bordered by Tussey Mountain south of Broad Top Mountain and Rocky Ridge north of Broad Top Mountain.

Allegheny Mountain (West Virginia – Virginia): Allegheny Mountain (spelling as Alleghany Mountain in Virginia) is a major mountain ridge in the southern range of the Allegheny Mountains, part of the Appalachian Mountains.  It forms the Eastern Continental Divide along part of its course and also serves as part of the Virginia-West Virginia state line.

Ridge-and-Valley Appalachians: The Ridge-and-Valley Appalachians, also called the Ridge and Valley Province or the Valley and Ridge Appalachians, are a physiographic province of the larger Appalachian division and are also a belt within the Appalachian Mountains extending from southeastern New York through northwestern New Jersey, westward into Pennsylvania and southward into Maryland, West Virginia, Virginia, Kentucky, Tennessee, Georgia and Alabama.  They form a broad arc between the Blue Ridge Mountains and the Appalachian Plateau physiographic province (the Allegheny and Cumberland Plateaus).  They are characterized by long, even ridges, with long, continuous valleys in between.

North Fork Mountain: North Fork Mountain is a quartzite-capped mountain ridge in the Ridge and Valley physiographic province of the Allegheny Mountains (or "High Alleghenies" or "Potomac Highlands") of eastern West Virginia, USA.  Kile Knob, at 4,588 feet (1,398 m), is the mountain's highest point, and Panther Knob and Pike Knob are nearly as high.

Supporting paragraphs: Broad Top Sideling Hill
Answer: Sideling Hill