Explanation: The correct answer is d. Flow cytometry for lymphocyte subsets. Flow cytometry is a diagnostic test used to analyze the quantity and types of lymphocytes in the blood, which helps identify abnormalities in the immune system and diagnose immunodeficiency disorders.

Incorrect choices:

a. A complete blood count (CBC) provides general information about the client's overall blood cell counts, but it does not specifically diagnose immunodeficiency disorders.

b. Urinalysis is used to assess kidney function and identify urinary tract infections but is not a specific test for diagnosing immunodeficiency disorders.

c. Skin testing for allergies is performed to identify allergic reactions to specific allergens and is not used to diagnose immunodeficiency disorders.