A useful check for this kind of question is to ask what would happen to the Assertion if the Reason were removed, since if the Assertion no longer has any legal basis without the Reason, then the Reason must be its explanation.
Because removing R would strip A of its legal foundation entirely, R is properly seen as the explanation for A, and both statements are true. The correct choice is that both (A) and (R) are true and (R) is the correct explanation of (A).
Another approach is to look at the broader purpose behind Order IX Rule 13 and check whether that purpose is what actually links the Assertion to the Reason.
Viewed through the lens of the rule's underlying purpose, (R) is not just an accompanying true fact but the actual mechanism that explains (A). The correct answer is that both (A) and (R) are true and (R) is the correct explanation of (A).