Final answer:
The word 'warm' matches best with the option (a) make feel good, as it relates to a comfortable level of heat and a sense of wellbeing.
Step-by-step explanation:
To match the word warm, we would be looking for something that correlates with giving a sensation of moderate heat or of comfort. Among the options provided (a) make feel good, (b) swelter, (c) enthusiasm, and (d) mumble, the one that best matches warm is (a) make feel good.
This is because warmth often denotes a comfortable level of heat and a sense of wellbeing or comfort. Whereas swelter implies an extreme heat that is often uncomfortable, enthusiasm relates more to intense and eager enjoyment or interest, and to mumble means to speak indistinctly or quietly, making it unrelated to the concept of warmth.