Which of the following ancient thinkers wrote treatises on grammar…

Concept

In the Indian grammatical tradition, a thinker qualifies here when a major work is devoted to Sanskrit grammar or the philosophy of language.

The relevant evidence is the work attributed to each thinker, not merely the thinker’s antiquity.

Application

• Pāṇini is the author of the Aṣṭādhyāyī, the foundational descriptive grammar of Sanskrit.

• Bhartṛhari composed the Vākyapadīya, a major work on Sanskrit grammar and the philosophy of language.

• Kapila is traditionally associated with Sāṃkhya philosophy, not with an extant grammatical treatise.

• Patañjali wrote the Mahābhāṣya, the major commentary on Pāṇini’s grammatical tradition.

Cross-check and contrast

• C and D only combines Kapila and Patañjali; Kapila’s association is with Sāṃkhya, while the set also omits Pāṇini and Bhartṛhari.

• A, C and D only includes Pāṇini and Patañjali but also Kapila, and it omits Bhartṛhari.

• A, B and D only groups Pāṇini, Bhartṛhari and Patañjali, each linked to a major grammatical or language-philosophy work.

• B and C only combines Bhartṛhari with Kapila and omits Pāṇini and Patañjali.

Result

Therefore, the thinkers are Pāṇini, Bhartṛhari and Patañjali.