Hey,

Any chord substend two angles on the circle. An acute angle in the larger section of the circle, since any chord divides the circle into two sections, and an obtuse angle in the smaller section.

When the chord passes through the center of the circle then the angles substended by the chord in both sections which are equal to 90 degrees.

Note that GRE figures are usually not to scale unless specifically mentioned.

Hence in the question the angle BAC can either be acute or obtuse! Hence D is correct option