The two half-lives are needed to reach 25% remaining. Therefore, the answer is 2.
We determine how many half-lives have passed in the rock sample, we can use the following logic:
In each half-life, the number of radioactive atoms remaining is halved.
If 25% of the original atoms remain, this means 75% have decayed.
Therefore, we need to find the number of half-lives it takes for 75% of the atoms to decay:
First half-life: 50% remaining (original 100% - 50% decay = 50%)
Second half-life: 25% remaining (50% - another 50% decay = 25%)