In this encounter, the correct coding order follows ICD-10-CM guidelines for coding multiple conditions when AIDS (B20) is documented, as it takes precedence. The patient's diagnosis of AIDS, documented with code B20, is reported as the primary diagnosis since it is a chronic condition. The M17.31 code is used to document unilateral primary osteoarthritis of the right knee, unrelated to AIDS but causing the patient's knee pain.

Explanation of each answer choice:

A. B20, M17.31: Correctly lists AIDS (B20) as the primary diagnosis and osteoarthritis of the right knee (M17.31) as the secondary diagnosis.

B. Z21, M08.861: Z21 represents asymptomatic HIV, not AIDS, which is incorrect here, and M08.861 is the code for juvenile idiopathic arthritis, not relevant in this case.

C. M17.11, B20: Incorrect as M17.11 is for unilateral primary osteoarthritis of the right knee, not left, and does not prioritize B20 as required.

D. M17.31, B20: Incorrect because it does not list B20 as the primary diagnosis, which is necessary per coding guidelines when AIDS is documented.

Thus, the correct answer is A. B20, M17.31.