You _______ touch animals in the zoo.

The correct answer is "mustn't". Here's why: * mustn't - means it is prohibited or not allowed to touch animals in the zoo. * must - means it is required to touch animals, which is not typically the case in a zoo. * can't - means it is not possible to touch animals, which may not be true in all zoos or for all animals. So, the completed sentence is: You mustn't touch animals in the zoo.