No, because lines being straight does not mean that the lines that intersect to form angle c are necessarily perpendicular. The figure does look like as if angle c is 90 degrees, but we cannot assume that, for all we know it could be 89 degrees or 91 degrees, or for that matter 89.99 degrees, which would visually be indistinguishable from two lines being perpendicular.

Vishesh says

Don’t we already know that angle c is 90 degrees since all lines are straight?

GMAT Quantum says

No, because lines being straight does not mean that the lines that intersect to form angle c are necessarily perpendicular. The figure does look like as if angle c is 90 degrees, but we cannot assume that, for all we know it could be 89 degrees or 91 degrees, or for that matter 89.99 degrees, which would visually be indistinguishable from two lines being perpendicular.