Answer
(a) $f_c = 100~Hz$
(b) $f_c = 100~Hz$
(c) $f_c = 200~Hz$
Work Step by Step
We can write an expression for the crossover frequency:
$f_c = \frac{1}{2\pi~R~C}$
(a) If the resistance is doubled then $f_c$ is decreased by a factor of $\frac{1}{2}$
$f_c = 100~Hz$
(b) If the capacitance is doubled then $f_c$ is decreased by a factor of $\frac{1}{2}$
$f_c = 100~Hz$
(c) The crossover frequency does not depend on the peak emf.
$f_c = 200~Hz$