Answer:
The correct answer is option c. viajeros.
Step-by-step explanation:
Traveller's check are values titles issued by a bank that used to be very popular before the credit cards appeared. Their purpose is to help people that travel to a place with a different currency to pay easily and safer than to transport cash, for this documents have the name of the holder, so in case of loss can be recovered, and in case of stealing can be ineffective. So the correct answer is c. travellers, for they are the one who use traveller's checks.
Option b. and c. are not agreeing with the question "who?" (¿quién?), and therefore they are incorrect.
Nevertheless, option a. dollars, would be incomplete for the traveller's checks covers all kinds of currencies, not only dollars. Actually, their purpose is precisely to help people that cannot get a specific currency to use these checks to travel without worrying about it.
Option b. passport, on the other hand, it is a requirement when paying with traveller's checks, in order to prove that you are the holder and therefore be able to use it. Although, the holder could use other official ID the passport it is the one universally accepted.