Step 1: Check the Reason first as a standalone biological fact.
Earthworms have no lungs or gills. Instead, oxygen and carbon dioxide cross directly through their thin outer skin, and this exchange only works while the skin surface stays moist and its surface network of blood capillaries stays active. That is exactly what Reason (R) claims, so R is true on its own.
Step 2: Work out what blotting paper actually does to the skin.
Blotting paper is designed to pull moisture away fast. Pressed against an earthworm, it strips the thin water film off the skin surface within a short time.
Step 3: Trace the consequence of that dryness.
Gases can only cross a respiratory surface once they dissolve in a surface film of water. Once that film is gone, oxygen can no longer dissolve and diffuse into the capillaries beneath the skin, so gas exchange collapses.
Step 4: Connect that collapse to the Assertion.
With no working gas exchange, the earthworm cannot get oxygen to its tissues, and it dies of suffocation. This is precisely the outcome Assertion (A) describes, so A is also true.
Step 5: Decide if R explains A.
Since the reason the worm dies (A) is exactly that its moist skin, the site of respiration described in R, was dried out, R is not just a coincidentally true statement, it is the actual mechanism behind A.
Final Answer:
Both A and R are true, and R correctly explains A, option (1).