The answer is D. Elections are often decided by only a few voters.
This is the best answer because this argument set the basis to explain the importance of every person's vote in a democratic government. Although one's vote might seem like it doesn't make a difference, it actually does, the election results are made of all the persons who relied on the voting system to express their political preference and make a difference. This is true especially in elections when candidates are strong because the elections tend to be decided by only a small difference (a few voters),
Besides, option A is incorrect because voting doesn't aim to symbolize an act, but to choose a leader who will express him/her ideology through the creation of policies that will determine a country's present and future.
Voting isn't less important than other types of civic engagement, it's equally important, if not more.
And voting is not mandatory by law, is a right guaranteed in the constitution that we all have as citizens, but it's not necessarily mandatory.