because an inscribed angle is equal to half its intercepted arc.
because inscribed angles that intercept the same arc are equal in measure.
because an inscribed angle in a semicircle always measures 90 degrees. It subtends 180 degrees and is equal to half of it. You don't have 90 degrees as a choice. Somebody messed up.
because arcs CD and AD make up a semicircle (180 degrees).