Which compound is a reusable, complex protein that speeds up chemical reactions? A) starch B) a sugar C) a lipid D) an enzyme

The answer is (d) an enzyme.

Enzymes are biological catalysts, which means they decrease the amount of activation energy need to start a reaction, therefore speeding up the rate of the reaction. They are part of the protein macromolecule group. Without them, our body would not process things fast enough for us to live.