If the probability a person is not a swimmer is $0.3$ then the probability a person is a swimmer
$p=1-0.3$
=$0.7$
$q=1-p=0.3$
Probability that out of $5$ persons $4$ are swimmers
=$p(X=4)$
=$c^{5}_{4}(0.7)^{4}(0.3)^{5-4}$
=$c^{5}_{4}(0.7)^{4}(0.3)$
Hence $'A'$ option is correct