Step 1:
Laghu, Guru, Yati, Virāma, and Gaņa are technical concepts used in Chandas-Śāstra.
Thus, Assertion (A) is correct.
Step 2:
“Pāvakā naḥ sarasvati” belongs to a Vedic metre, not Laukika Chandas.
Hence, Reason (R) is incorrect.
Step 3:
Vedic and Laukika metres are classified differently in Sanskrit prosody.
Step 4:
Therefore:
\[
\boxed{\text{A is correct but (R) is not correct}}
\]
Hence, the correct option is:
\[
\boxed{\text{(3)}}
\]