Explanation: The correct answer is c. Serum immunoglobulin levels. A primary immunodeficiency disorder often results in abnormal levels of immunoglobulins (antibodies) in the blood. Measuring serum immunoglobulin levels can help identify potential deficiencies and guide the diagnosis of primary immunodeficiency disorders.

Incorrect choices:

a. Enzyme-linked immunosorbent assay (ELISA) is a test used to detect specific antibodies or antigens in the blood, but it is not specific to diagnosing primary immunodeficiency disorders.

b. A chest X-ray is not a specific test for diagnosing primary immunodeficiency disorders. It is typically used to assess the condition of the lungs and chest organs.

d. A complete blood count (CBC) is a general blood test that may reveal abnormalities in white blood cell count but does not specifically assess immunoglobulin levels.