Step 1: The question refers to extramammary Paget's disease (EMPD), an uncommon, slow-growing, usually non-invasive intraepithelial adenocarcinoma that arises in skin rich in apocrine glands outside the breast.
Step 2: In the female genital tract the classic site of extramammary Paget's disease is the vulva. It presents as a chronic, eczematous, red and white plaque that is often itchy and resistant to topical treatment, so vulva is the correct association.
Step 3: The vagina (option b), cervix (option c) and uterine corpus (option d) are lined predominantly by glandular or squamous mucosa rather than apocrine-bearing skin and are not the recognised sites of Paget's disease. Hence vulva is the answer.