Let the projection of p on the plane ABC be q, and then q is the center of the circumscribed circle of △ABC.

∫∠ABC = 90°

∴AC is the diameter of circle Q, and Q is the midpoint of AB.

Even if OP must pass through point q and OP⊥ plane ABC.

On the PAOC plane, PA=PC=OA=OC=OQ=2.

∴AC=2√3,OQ= 1

After B, BD⊥AC is in D. Obviously, when B moves to make D coincide with Q, BD obtains the maximum value of AC/2=√3.

v(O-ABC)= S(△ABC)OQ/3 = AC BD OQ/6≤2√3×√3× 1/6 = 1

Choose B.