Intransitive Verbs
An intransitive verb is a verb that does not require a direct object to complete its meaning. It expresses action or a state of being without transferring that action to an object. In other words, the subject of the sentence performs the action, but there is no recipient or object affected by the action.

Explanation
Out of the given options, the intransitive verb is cry. Let's analyze each option to understand why 'cry' is the correct answer.

a) Cry: This verb does not require a direct object to complete its meaning. It expresses an action performed by the subject (someone shedding tears) without affecting any object. For example, "She cried during the sad movie."

b) Dance: Although dance can be used transitively in some contexts (e.g., "She danced the tango"), it can also be used intransitively. When used without an object, it expresses an action performed by the subject without affecting any object. For example, "They danced all night at the party."

c) Sing: Similar to 'dance,' 'sing' can be used transitively and intransitively. When used intransitively, it expresses an action performed by the subject without affecting any object. For example, "He sings beautifully."

d) Cook: Cook is a transitive verb as it requires a direct object to complete its meaning. It expresses an action performed by the subject (preparing food) that affects an object (the food being cooked). For example, "She cooked dinner for her family."

Therefore, the correct answer is option 'a' - cry. It is the only verb among the given options that is always used intransitively, expressing an action performed by the subject without affecting any object.