Synonyms can be:

Synonyms are words that have similar meanings, and they can exist across different parts of speech, including nouns, verbs, and adjectives. For example, the noun "vehicle" has synonyms like "car," "truck," and "automobile." In the context of verbs, consider "run," which can be synonymous with "jog," "dash," or "sprint." Similarly, the adjective "happy" has synonyms such as "joyful," "cheerful," and "content."

The idea that synonyms are restricted to only one part of speech does not hold true, as the richness of language allows various synonyms to apply across the grammatical categories. Therefore, it is accurate to conclude that synonyms can include nouns, verbs, and adjectives, making the answer to the question comprehensive.