Concept:
Assertion-Reason questions require checking factual correctness and explanatory relation.
Step 1: Examine Assertion (A).
The definition given for preposition is correct.
A preposition shows relation between a noun/pronoun and another word.
Hence, Assertion (A) is true.
Step 2: Examine Reason (R).
The statement:
\[
a,\; an,\; the
\]
are articles is also correct.
Hence, Reason (R) is true.
Step 3: Check whether (R) explains (A).
The explanation about articles does not explain the definition of a preposition.
Thus, both statements are correct but unrelated in explanation.
Step 4: Conclusion.
Therefore:
\[
\boxed{\text{Both (A) and (R) are correct but (R) is not the correct explanation of (A).}}
\]