Step 1: Understanding the Concept:
The question asks to identify the most common metrical form (\textit{chanda}) used in Sanskrit epic poetry (like the Rāmāyaṇa and Mahābhārata) and other narrative classical texts.
Step 2: Detailed Explanation:
The most ubiquitous metre in Sanskrit is the Anuṣṭubh. This metre is also commonly known as the Śloka. It is a relatively simple and versatile metre, making it ideal for narration.
Its structure is:
Four lines (\textit{pādas}).
Each line has eight syllables.
The vast majority of the verses in the Mahābhārata, the Rāmāyaṇa, the Purāṇas, and the Dharmaśāstras are composed in the Anuṣṭubh metre.
The other metres listed are more complex and are typically used in ornate court poetry (\textit{kāvya}) rather than for long narratives:
Jagatī: A metre with 12 syllables per line.
Indravajrā: A specific type of Triṣṭubh metre with 11 syllables per line.
Sragdharā: A very long and complex metre with 21 syllables per line.
Step 3: Final Answer:
The Anuṣṭubh is the most frequently employed metre in Sanskrit epics and classical literature. Therefore, option (1) is correct.