The correct structure of histidine in a strongly acidic solution (pH=2) is:

Step 1: Understanding Histidine's Groups — Histidine consists of a carboxyl group, an amino group, and an imidazole ring side chain.

Step 2: Protonation at pH 2 — At highly acidic pH levels ($\\text{pH} \\approx 2$): (1) The amino group is protonated ($\ce{-NH3^+}$), (2) The imidazole ring's basic nitrogen is protonated, and (3) The carboxyl group exists as $\ce{-COOH}$.

Step 3: Conclusion — Structure (c) correctly displays the fully protonated form where both amino and imidazole nitrogens carry positive charges. Correct option is (c).

Remember: The $\text{p}K_a$ of the histidine side chain imidazole group is approximately 6.0.