All other answer choices involve the variable 'b' and/or are tied to it in some way. The sole number 4 has no connection to the variable b, therefore it is C. 4.
The probability of finding 2 never married 10c2*0.4^2*0.6^8= the probability of finding 1 never married 10C1*0.4*0.6^9= the probability of finding 0 never married 10C0*0.6^10=
the answer is all the above answers added together. think you can do the rest of the calculation by yourself. :)