(A) : A person hears maximum sound at displacement anti node (or) pressure node.
(B) : Two organ pipes of same length open at both ends produce sound of different pitch if their radii are different.
(C) : If oil of density higher than water is filled in place of water in a resonance tube, its frequency remain unchanged.

Due to end correction, the frequency of an open organ pipe depends on its radius (r) and is given by $f=\frac{v}{2(L+1.2r)}$. So, the pitch will be different for different radii.

Perception of sound is due to pressure variations that is maximum at pressure nodes.

Frequency of standing wave in resonance tube does not depend on the density of liquid used.