You might be quick to think the answer is something like hydrogen and oxygen because both form to make water. But understand that the question is not asking about important elements in life, just which element makes up organic molecules. This element happens to be carbon.
It's important to understand that carbon is lucky in that it has 4 valence electrons and is able to bond with other important elements, like F, N, and especially H. The bond between C-H is essential in organic chemistry because it represents the structure of an organic molecule and helps with the IUPAC naming of organic molecules. It also suffices to say that there is a cycle for carbon in the carbon cycle, which transports carbon from one place in our world to the other.
So, it suffices to say that carbon is in fact essential for making up organic molecules.
