Bedd Arthur or Arthur's Grave is a possible Neolithic hengiform monument megalithic site in the Preseli Hills in the Welsh county of Pembrokeshire.
 Thirteen upright stones and at least 2 fallen ones, each around 0.6 m high for an oval horseshoe with similarities to the earliest form of Stonehenge.
Amesbury is a town and civil parish in Wiltshire, England.
 It is most famous for the prehistoric monument of Stonehenge which is in its parish, and for the discovery of the Amesbury Archer—dubbed the King of Stonehenge in the press—in 2002.
 It has been confirmed by archaeologists that it is the oldest continuously occupied settlement in the United Kingdom, having been first settled around 8820 BC.
Cairn Lee is a prehistoric monument in Aberdeenshire, Scotland.
 Cairn Lee and proximate Longman Hill are the oldest prehistoric features in the local area.
Avebury ( ) is a village and civil parish in Wiltshire, England.
 The village is about 5.5 mi west of Marlborough and 8 mi northeast of Devizes.
 Much of the village is encircled by the prehistoric monument complex also known as Avebury.
 The parish also includes the small villages of Avebury Trusloe and Beckhampton, and the hamlet of West Kennett.
Winterbourne Stoke is a village and civil parish in Wiltshire, England, about 5 mi west of Amesbury and 3 mi west of the prehistoric monument of Stonehenge.
The Fishguard Volcanic Group is an Ordovician lithostratigraphic group (a sequence of rock strata) in west Wales.
 The name is derived from the town of Fishguard in Pembrokeshire.
 This assemblage of rocks has also been referred to as the Fishguard Volcanic Series or Fishguard Volcanic Complex.
 These rocks are believed to be the source of the 'bluestones' which form a part of the well-known prehistoric monument of Stonehenge in southern England.
The Vinča culture, also known as Turdaș culture or Turdaș-Vinča culture, is a Neolithic archaeological culture in Central Europe and Southeastern Europe, dated to the period 5700–4500 BC.
 Named for its type site, Vinča-Belo Brdo, a large tell settlement discovered by Serbian archaeologist Miloje Vasić in 1908, it represents the material remains of a prehistoric society mainly distinguished by its settlement pattern and ritual behaviour.
 Farming technology first introduced to the region during the First Temperate Neolithic was developed further by the Vinča culture, fuelling a population boom and producing some of the largest settlements in prehistoric Europe.
 These settlements maintained a high degree of cultural uniformity through the long-distance exchange of ritual items, but were probably not politically unified.
 Various styles of zoomorphic and anthropomorphic figurines are hallmarks of the culture, as are the Vinča symbols, which some conjecture to be the earliest form of proto-writing.
 Though not conventionally considered part of the Chalcolithic or "Copper Age", the Vinča culture provides the earliest known example of copper metallurgy.
Newgrange (Irish: "Sí an Ḃrú" or "Brú na Bóinne") is a prehistoric monument in County Meath, Ireland, located 8 km west of Drogheda on the north side of the River Boyne.
 It was built during the Neolithic period, around 3200 BC, making it older than Stonehenge and the Egyptian pyramids.
 The site consists of a large circular mound with an inner stone passageway and chambers.
 Human bones and possible grave goods or votive offerings were found in these chambers.
 The mound has a retaining wall at the front, made mostly of white quartz cobblestones, and is ringed by engraved kerbstones.
 Many of the larger stones of Newgrange are covered in megalithic art.
 The mound is also ringed by a stone circle.
 Some of the material that makes up the monument came from as far away as the Mournes and Wicklow Mountains.
 There is no agreement about what the site was used for, but it is believed that it had religious significance.
 Its entrance is aligned with the rising sun on the winter solstice, when sunlight shines through a 'roofbox' and floods the inner chamber.
 Several other passage tombs in Ireland are aligned with solstices and equinoxes, and Cairn G at Carrowkeel has a similar 'roofbox'.
 Newgrange also shares many similarities with other Neolithic constructions in Western Europe, such as Maeshowe in Orkney, Scotland and Bryn Celli Ddu in Wales.
 It is the most famous monument within the Neolithic Brú na Bóinne complex, alongside the similar passage tomb mounds of Knowth and Dowth, and as such is a part of the Brú na Bóinne UNESCO World Heritage Site.
Stonehenge is a prehistoric monument in Wiltshire, England, 2 mi west of Amesbury and 8 mi north of Salisbury.
 It consists of a ring of standing stones, with each standing stone around 13 ft (4.1 metres) high, 6 ft 11 in (2.1 metres) wide and weighing around 25 tons.
 The stones are set within earthworks in the middle of the most dense complex of Neolithic and Bronze Age monuments in England, including several hundred burial mounds.
Larkhill is a garrison town in the civil parish of Durrington, Wiltshire, England.
 It lies about 1+3/4 mi west of the centre of Durrington village and 1+1/2 mi north of the prehistoric monument of Stonehenge.
 It is about 10 mi north of Salisbury.

Question: What earliest form of prehistoric monument in Wiltshire, England was Bedd Arthur similar to?
Answer: Stonehenge