The correct answer is $\boxed{\frac{1}{2}}$.
Player X has a $\frac{1}{2}$ chance of winning on the first toss. If he does not win on the first toss, then player Y has a $\frac{1}{2}$ chance of winning on the first toss. In this case, player X has a $\frac{1}{2}$ chance of winning on the second toss. This process continues until one player wins. The probability that player X wins is the sum of the probabilities of winning on each toss, which is $\frac{1}{2} + \frac{1}{2} \cdot \frac{1}{2} = \frac{1}{2}$.
Option A is incorrect because it is the probability that player X loses the game. Option B is incorrect because it is the probability that player X wins the game if he starts and player Y tosses the coin first. Option C is incorrect because it is the probability that player X wins the game if he starts and player Y tosses the coin second. Option D is incorrect because it is the probability that player X wins the game if he starts and player Y tosses the coin third.