Kasundi is a tangy mustard-based paste, popular in Bengali cuisine. It is made primarily from mustard seeds and often used as a condiment for fish and other dishes.
- Coriander seeds are used in various spice blends but are not the base for Kasundi.
- Fenugreek seeds have a distinct flavor and are used in many Indian dishes, but they are not the key ingredient in Kasundi.
- Nutmeg is a sweet spice and is not a primary ingredient in Kasundi.
Thus, the correct answer is Mustard seeds.