The best answer to this question is C. Political action committees, or PACs, give mostly to incumbents who are seeking reelection. This may, however, be changing.
In the past, PACs have preferred to donate money to incumbents. There is a very good reason for this. Incumbents have had a tremendous advantage in getting reelected. This means that a donation to an incumbent is most likely to be a donation to the winning candidate. PACs tend to want to have access to the person who actually wins, so they tend to give to the incumbent because the incumbent is generally likely to win. PACs will often not want to waste money backing a challenger and they will not want to antagonize an incumbent by giving to the challenger.
This may, however, be changing. The new rules in our campaign finance system since the Citizens United decision allow PACs to raise and spend all the money they want as long as it is not coordinated with any candidate. This meant that we saw in the 2012 election huge campaigns by PACs for or against particular candidates. More of these campaigns were anti-incumbent than usual. It is too early to know if this will become a trend, but it is something to watch in years to come.