Step 1: Understand the question.
There is an Assertion (A) and a Reason (R) about setting aside an ex parte decree under the Code of Civil Procedure, 1908. We must judge if both are true and if R explains A.
Step 2: Understand an ex parte decree.
An ex parte decree is one passed against a defendant who did not appear when the suit was called, even though the suit was heard. Such a decree stands unless set aside by law.
Step 3: Test the Assertion.
Assertion A says the decree may be set aside if the defendant shows he was prevented by sufficient cause from appearing. This is a correct ground under the law, so A is true.
Step 4: Test the Reason.
Reason R says that under Order IX Rule 13 CPC, the court must be satisfied that summons was not duly served or that sufficient cause prevented appearance. This reproduces the actual rule, so R is true.
Step 5: See the link between A and R.
The Assertion states one ground; the Reason gives the statutory provision (Order IX Rule 13) that contains exactly that ground. So R directly explains why A is correct.
Step 6: Reach the answer.
Both A and R are true, and R correctly explains A.
\[ \boxed{\text{Both (A) and (R) are true and (R) is the correct explanation of (A).}} \]