Which chemical is the building block of amino acids and proteins?

oxygen
carbon
nitrogen
hydrogen

I believe that hydrogen is the basic building block 

Carbon is the building block of amino acids. The first thing you might be asking is, "What is an amino acid?" There are more than fifty, and each one of them is a little different. Amino acids are used in every cell of your body to build the proteins you need to survive. All organisms need some proteins, whether they are used in muscles or as simple structures in the cell membrane. Even though all organisms have differences, they still have one thing in common: the need for basic chemical building blocks.

Structure of an amino acid Amino acids have a two-carbon bond. One of the carbons is part of a group called the carboxyl group (COO-). A carboxyl group is made up of one carbon (C) and two oxygen (O) atoms. That carboxyl group has a negative charge, since it is a carboxylic acid (-COOH) that has lost its hydrogen (H) atom. What is left — the carboxyl group — is called a conjugate base. The second carbon is connected to the amino group. Amino means there is an NH2 group bonded to the carbon atom. In the image, you see a "+" and a "-". Those positive and negative signs are there because, in amino acids, one hydrogen atom moves to the other end of the molecule. An extra "H" gives you a positive charge.