Explanation:

- Correct Answer: Grammar

- The *Ashtadhyayi* is an ancient Indian text composed by Sanskrit grammarian Panini, which systematically and comprehensively describes the grammar of Sanskrit through a set of rules and sutras. It is regarded as one of the most significant works in the field of linguistics.

- Option 1: Medicine

- This option is incorrect as the *Ashtadhyayi* focuses on linguistic rules and has no relevance to medicine.

- Option 2: Law

- This option is incorrect because *Ashtadhyayi* does not cover legal topics or principles; it is specifically a grammatical treatise.

- Option 3: Economy

- This option is incorrect since the *Ashtadhyayi* deals only with the structure and function of language, not economic theories or principles.

Overall, Panini's work in the *Ashtadhyayi* is fundamental in the study of Sanskrit grammar, influencing grammar study processes not just in India but also globally.