Why isn't "B" the answer?
If they had to all three be heads, then the calculation would be 1/2 * 1/2 * 1/2 = 1/8
In addition to getting the chosen outcome if we get three heads, we know that also we have success if the the three are all tails. So right there you know that the probability is greater than the 1/8 for three heads.
How much greater?
Well we have success if they are all heads. The probability of that is 1/8.
We also have success if they are all tails. The probability of that is also 1/8.
So we add the two together for the total probability of success, which is 1/8 + 1/8 = 1/4, choose