Which of the following is an example of a modal verb?

Modal verbs are a specific type of auxiliary verb used to express ability, possibility, permission, or obligation. They help to modify the main verb in a sentence by adding nuances related to how certain or likely an action is to occur.

In this instance, "can" is indeed a modal verb because it indicates ability or possibility. For instance, in the sentence "She can swim," "can" shows the ability of "she" to perform the action of swimming.

The other choices (running, walk, and think) do not function as modal verbs. They are base forms of action verbs or gerunds, and they do not convey the same kind of modality that "can" does. For example, "running" and "walking" describe actions, while "think" refers to the act of contemplating or processing information, lacking the modality aspect central to modal verbs. Thus, "can" is the correct answer as it uniquely fits the definition of a modal verb.