* Rohit: This is a proper noun, referring to a specific person.
* helped: This is a verb.
* his: This is a possessive pronoun.
* mother: This is a common noun, referring to a female parent.
* cook: This is a verb.
* the: This is a definite article (not a noun).
* dinner: This is a common noun, referring to a meal.
Therefore, there are three nouns in the sentence: Rohit, mother, and dinner.