Question

why is butter solid and mustard oil is liquid at room temperature

Answer 1

Butter has a higher proportion of straight (i.e. hydrocarbon chains are unbent) saturated fatty acids (all carbons bonded to each other by single bonds), which can pack together very tightly, and thus have a higher melting point (i.e. they will be solid at a higher temperature). As a plant oil, mustard seed oil is higher in unsaturated fatty acids (some carbons connected via carbon-carbon double bonds), which pack together in a "looser" fashion than do saturated FA's. Unsaturated FAs tend to bend or kink around those double bonds--this makes them less stable at a given temperature, and thus they have lower melting points (i.e. they remain liquid at lower temperatures).
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Answer 2

Animal fats have saturated fatty acids which pack together by weak interactions and butter is an animal fat.

The interactions in plant fats aren't stable because they have kinks in their fatty acid tails and mustard oil is a vegetable oil and has plant fats.

This is the reason that butter is solid and mustard oil is liquid at room temperature.