Answer
In situation $d$, there is the possibility that strings A and B are oscillating at the same resonant frequency.
Work Step by Step
We can write a general expression for the resonant frequency:
$f = \frac{v}{\lambda} = \frac{1}{\lambda}\sqrt{\frac{\tau}{\mu}}$
Since the tension $\tau$ is greater in string $b$ than string $a$, the two strings can have the same resonant frequency only if the wavelength in string $b$ is longer than the wavelength in string $a$.
Only situation $d$ satisfies this condition.
In situation $d$, there is the possibility that strings A and B are oscillating at the same resonant frequency.
