An expression is shown below?

42 = 2*3*7, these are the Prime factors of 42. The other factorisations are formed by grouping these prime factors: 6*7, 14*3, 21*2

Take A)35 for example
35 = 5*7

42 and 35 have 7 as a common divisor. So, for x=35 the expression would be:
sqrt(42 * 35) = sqrt(6 * 7 * 5 * 7) or sqrt(7² * 30) =
sqrt(49 * 30) = 7 * sqrt(30)

which would be the simplified form.

The only number in the given list, which has no divisor with 42 in common, is 23. So,
B)23
is your answer.