Step 1: Understand the format.
We have an Assertion (A) and a Reason (R). We must check if both are true, and if R correctly explains A.
Step 2: Read the Assertion.
A says an ex parte decree (a decree passed when the defendant did not appear) can be set aside if the defendant shows he had a good enough reason for not appearing. This is correct law.
Step 3: Read the Reason.
R says that under Order IX Rule 13 of the Code of Civil Procedure, 1908, the court must be satisfied either that the summons was not properly served, or that the defendant was kept away by sufficient cause. This is also correct.
Step 4: See if R explains A.
A states the ground (sufficient cause). R gives the exact rule and conditions, including sufficient cause, that allow setting aside the decree. So R supplies the legal basis for A. That means R correctly explains A.
Step 5: Pick the matching option.
Both A and R are true, and R is the correct explanation of A. The options that say one is false, or that R does not explain A, are wrong.
Step 6: State the answer.
Both are true and R explains A.
\[ \boxed{\text{Both (A) and (R) are true and (R) is the correct explanation of (A).}} \]