(D) Assertion (A) is false, but Reason (R) is true.
The Assertion is false because selecting any number from 1 to 20 is a sure event with P(E) = 1, not selecting a specific number (which would be 1/20). The Reason is true: P(E) = 1 implies a sure event.
The Assertion is ambiguous but as stated — "probability of selecting a number at random from 1 to 20 is 1" — it appears to claim a specific selection has probability 1, which is false (it would be 1/20). However, if interpreted as selecting any number from the set, it is actually a sure event (P = 1). CBSE typically treats the Assertion here as false (implying a specific number), making option (D) correct. The Reason is directly from the textbook and is true. Focus on the distinction: sure event → P = 1; specific number → P = 1/20.