Final answer:
Modal is not a separate part of speech; it is a type of auxiliary verb included in the category of verbs. Adjectives, adverbs, and verbs are parts of speech with functions of describing, modifying, and expressing actions or states.
Step-by-step explanation:
The student's question revolves around parts of speech, which are fundamental categories of words with different functions and roles in sentences. In English grammar, the main parts of speech are nouns, pronouns, verbs, adjectives, adverbs, prepositions, conjunctions, and interjections. Among the options provided, adjective, adverb, and verb are acknowledged as parts of speech that describe, modify, and express actions or states, respectively. Modal is not considered a separate part of speech; rather, it is a type of auxiliary verb. Modals, such as 'can', 'will', 'should', and 'must', are used to express necessity, possibility, permission, and other conditions.
To determine which part of speech a word belongs to, examine its function in the sentence. Adjectives modify nouns by giving more information about their qualities, quantity, or features. Adverbs modify verbs, adjectives, or other adverbs, often relating to manner, place, time, frequency, degree, or level of certainty. Verbs depict actions, occurrences, or states of being.
Given the options presented, the final answer is C. Modal. It is not considered a part of speech on its own but fits under the category of verbs. Therefore, when identifying nouns, adjectives, verbs, and adverbs, one should include the entire phrase that these parts of speech comprise as they may appear in more complex forms beyond single words.