Match List-I with List-II and select the correct answer:

List-I (Radioisotope)	List-II (Diagnostic use)
a. Arsenic-74	1. Activity of thyroid gland
b. Cobalt-60	2. Blood disorder
c. Iodine-131	3. Tumour
d. Sodium-24	4. Cancer

* Arsenic-74: This radioisotope is primarily used to locate tumors within the body.

* Cobalt-60: Cobalt-60 is a strong radioactive source used in radiation therapy to treat cancer. It works by destroying cancerous cells.

* Iodine-131: This radioisotope is readily absorbed by the thyroid gland. Therefore, it's used to measure the activity of the thyroid gland and diagnose thyroid disorders.

* Sodium-24: This radioisotope is used to study blood disorders, particularly issues related to blood circulation.

Therefore, the correct matching is:
a - 3 (Arsenic-74 - Tumour)
b - 4 (Cobalt-60 - Cancer)
c - 1 (Iodine-131 - Activity of thyroid gland)
d - 2 (Sodium-24 - Blood disorder)

So, the correct answer is Option C: a-3, b-4, c-1, d-2