Step 1:
Mrgaśirā is one of the 27 Nakshatras in Indian astronomy.
Thus, Assertion (A) is correct.
Step 2:
The sequence of Nakshatras indeed places Mrgaśirā as the fifth Nakshatra.
Step 3:
The Reason correctly supports and explains the Assertion.
Step 4:
Hence:
\[
\boxed{\text{Both (A) and (R) are correct and (R) correctly explains (A)}}
\]
Therefore, the correct answer is:
\[
\boxed{\text{(1)}}
\]