Answer is B
$n(A\cup B) = n(A)+n(B)-n(A\cap B)$
n(persons who do not prefer either chicken or fish) = n(persons who do not prefer chicken) + n(persons who do not prefer fish) – n(persons who do not prefer both chicken and fish)
n(persons who do not prefer either chicken or fish) = (100 – 60) + 50 – 10 = 80
ׯ The number of persons who prefer both fish and chicken = 100 – 80 = 20