Option 1 states that both the assertion (A) and reason (R) are true, and that R is the correct explanation of A. Let`s analyze each statement separately:

Assertion (A): Elephantiasis is associated with swollen legs and feet.

This statement is true. Elephantiasis is a condition characterized by the extreme enlargement of body parts, such as the legs and feet. It is caused by the obstruction of lymphatic vessels by filarial parasites, which results in the accumulation of fluid and subsequent swelling.

Reason (R): Lymph nodes are blocked by filarial parasite resulting in accumulation of fluid.

This statement is also true. Elephantiasis is primarily caused by filarial parasites, specifically Wuchereria bancrofti, Brugia malayi, and Brugia timori. These parasites are transmitted to humans through the bites of infected mosquitoes. Once inside the body, the parasites migrate to the lymphatic vessels and block the flow of lymph fluid, leading to its accumulation and subsequent swelling.

Therefore, in this case, both the assertion and the reason are true, and the reason correctly explains the assertion. Hence, option 1 is the correct answer.