A manufacturing company based in Country R.

Sales in S$ and T$

Purchases in U$

Profit margin measured in R$

To improve or maintain operating margin in R$ terms:

You want costs in R$ to fall and/or revenues in R$ to rise.

Strengthening of the R$ against U$ means each U$ costs fewer R$, so purchase costs fall in home-currency terms → margin improves.

Weakening of customer currencies (S$ or T$) against R$ would reduce revenue in R$, harming the margin.

Weakening of the R$ against U$ would make U$ costs more expensive.

So the helpful change is: R$ strengthens against U$ → option C.