Correct Answer - D
Let X denotes the number of coins showing heads up. Then, X is a binomial variate with n=100 and probabilituy of success p.
We have
P(X=51)=P(X=50)
⇒.100C51p51q49=.100C50p50q50, where q=1-p
⇒pq=.100C50.100C51=5150⇒p1-p=5150⇒p=51101