The correct answer here would be B.
The withdrawal method does not always work and accidents can happen. A diaphragm can move or come out and it's percentage of effectiveness is not 100% guaranteed, it is more around 92 - 96%. An IUD is 99% effective as a contraception and is one of the most reliable forms. However to be 100% sure you will not get pregnant is to practice abstinence which means not having sexual intercourse at all.