The answer is 75 and here's why. 
Let's figure out the amount of remaining votes to prove this. 
250 total votes - 150 votes for candidate a = 100 votes. 
 minus ↑ sign 
Candidate B received 25% of the votes, so let's find 25% of 100 
100 * 0.25 = 25 votes
Candidate B only got 25 votes (that's kinda sad, poor guy) 
100 - 25 votes = the # of votes candidate C got 
Candidate C got 75 votes!