Final answer:
The correct definition of a verb is option D, an action word that describes what the noun is doing. Verbs are central to expressing actions, states, or occurrences in the predicate of a sentence and can require objects and adverbials.
Step-by-step explanation:
A verb is a word that defines an action, state, or occurrence, and forming the main part of the predicate of a sentence. The essence of a verb is to depict activities, processes, conditions, or events. Typically, a verb will require a subject, which is who or what is performing the action, and may also include objects and adverbials. For instance, 'She runs' - 'She' is the subject, and 'runs' is the verb indicating what she is doing.
Answering your question, the correct choice is:
D.) A verb is an action word that describes what the noun is doing.
However, the definition of a verb extends beyond just action; it can also introduce states or descriptions, as in 'He seems happy' where 'seems' is a linking verb linking the subject 'He' with the subject complement 'happy'. Linking verbs can be completed by subject complements, which are noun phrases (NPs) or adjectives (adjs) that describe or identify the subject. In addition, verbs may change form to indicate different tenses, expressing actions or states at various times.
As for types of verbs, transitive verbs require a direct object, such as in 'She reads a book,' where 'reads' is the transitive verb and 'a book' is the direct object receiving the action. In short, verbs are a crucial part of speech in the English language, vital for describing the dynamics within a sentence.