To solve this question, we need to identify which multilateral negotiation was signed between India and the Eurasian Economic Union (EAEU) in August 2025 as a step towards a Free Trade Agreement (FTA). Let's evaluate the options provided:

1. India-EAEU Terms of Reference (ToR) for an FTA: The 'Terms of Reference' (ToR) usually outline the guidelines, scope, and details essential to start formal negotiations for agreements like FTAs. It makes sense that this would precede an actual Free Trade Agreement, setting the path for further detailed negotiations.
2. India-EAEU Cultural Exchange Agreement: While cultural exchanges are commonly agreed upon between nations, they do not directly relate to FTAs, which are primarily concerned with trade-related negotiations and agreements.
3. India-EAEU Defence Pact: Defence pacts involve military collaboration and are unrelated to trade negotiations, meaning they are unlikely to be part of the process towards an FTA.
4. India-EAEU Comprehensive Economic Partnership Agreement: Although comprehensive economic partnerships expand beyond just trade, encompassing investment and other economic collaborations, they are usually at the same level as FTAs rather than a precursor. The question specifies a step towards an FTA, which fits the profile of a ToR.

Based on the above analysis, the correct choice is the option India-EAEU Terms of Reference (ToR) for an FTA. This ToR serves as the foundational agreement marking the commencement of formal FTA discussions between India and the EAEU.

In essence, signing a Terms of Reference is an initiating measure in international negotiations. It effectively sets the agenda and parameters for future comprehensive trade and economic discussions.