Why
This tests Courage: keeping professional boundaries with the people you serve, and keeping the public’s confidence through the way you conduct yourself. A gift offered because of your role can create, or look like, an obligation to give something back. Good looks like turning the offer down politely, explaining why, and paying, so the goodwill is kept without any suggestion of favour.
A (Counterproductive): it links a free gift to a promise of extra police attention. That is exactly the kind of favour-for-favour arrangement that damages trust, and the customers watching would see it.
B (Most effective): it is courteous and clear. The owner’s goodwill is kept, the reason is explained, and everyone watching sees that officers pay their way.
C (Ineffective): it avoids a moment of awkwardness but still accepts a gift given because of your role. Small, repeated freebies can build an expectation that the giver will be treated differently.
D (Effective): paying keeps the professional boundary, which matters most. Avoiding the café is unnecessary, though, and misses the chance to explain to the owner why officers do not accept free goods.