To answer the above question, the relevant sentence is: The highest flow velocities are found at the surface, representing the sum of the velocities of all the layers below.
This justifies what answer for what question? Q & A:
Passage: The lowest velocities are near the base of the glacier and along valley sides where friction acts against flow, causing the most deformation. Velocity increases inward toward the center line and upward, as the amount of deformation decreases. The highest flow velocities are found at the surface, representing the sum of the velocities of all the layers below.

Question: Based on this passage, in a cross-section of a glacier the greatest flow velocities (speeds) occur?
at the surface