A question about the exchange economy
Hint:

These exercises seem to be encouraging you to think about the First Welfare Theorem, and whether it will hold. Recall the theorem which states that if markets are competitive (price takers), markets are complete (full information), and that consumers are locally non-satiated, then every competitive equilibrium should be Pareto optimal.

For each sub-question, there is at least one agent whose preferences are not locally non-satiated. For part (b), you are correct that there will be no trade. That is, you are at a competitive equilibrium. You should discuss if the allocation is Pareto optimal. If the allocation is PO, prove it, and if not, show a counterexample.

For part (c), you do not need to know the utility functions of each agent to do this question. Since we only care about utility up to a monotonic transformation, you can in theory make up a utility function that fits agent A's description. In this case, all three drinks are perfect substitutes, so you can express agent A's utility as

$$u_A(m, j, w) = m + j +w$$

Or any monotonic transformation of that. But this is not necessary. You can solve (c) with just the definition of a Pareto optimal allocation and some intuition. Hope this helps.