a
(a)
Total length of a pendulum remains same, so extreme point $D$ lies on the line $A B$, as shown below.
This can be proved by applying energy conservation between extreme positions $A$ and $D$ (its given friction is abscent),
$K_A+U_A=K_B+U_B=K_D+U_D$
$\Rightarrow 0+U_A=0+U_B=0+U_D$
$\Rightarrow U_A=U_B=U_D \Rightarrow h_A=h_B=h_D$
