Step 1: Understand the structure of natural rubber.
Natural rubber is a polymer made from the monomer isoprene, which is a diene compound. The polymerization of isoprene molecules results in the formation of polyisoprene, commonly known as natural rubber.
Step 2: Identify the correct monomer.
The monomer of natural rubber is isoprene (\( \text{C}_5\text{H}_8 \)).
Thus, the correct answer is (A).