b
$v =$ speed of sound in water $=1500 m / s$
frequency received by $A = f$
$=\left[\frac{ V - V _{ A }}{ V - V _{ B }}\right] f _{0}=\left[\frac{1500-5}{1500-7.5}\right] f _{0}$
frequency received by $B = f ^{\prime \prime}=$
$\left[\frac{ V + V _{ B }}{ V + V _{ A }}\right] f ^{\prime}=\left[\frac{1500+7.5}{1500+5}\right]\left[ \frac{1500-5}{1500-7.5}\right] f$
$f ^{\prime \prime}=\left(\frac{1500+7.5}{1500-7.5}\right)\left(\frac{1500-5}{1500+5}\right)(500)$
$=502\; Hz$
