Instructions: In this task, you are given a multiple-choice question about healthcare. Answer the question based on your information and classify your answers into '1', '2', '3', and '4'.
Input: Question: Histidine is characterized by:
Options: <1> Present an indole group in its structure. <2> Present a unique N in its structure. <3> A side chain of aromatic character. <4> A side chain that can take or release protons at physiological pH. <5> Being a protein involved in inflammation.
Output:
4