Final answer:
The common noun in the phrase 'My car is red' is 'car'. A common noun is a general word for a person, place, thing, or idea. It isn't capitalised and doesn't refer to something specific.
Step-by-step explanation:
In the sentence you provided, 'My car is red,' the common noun is 'car'. A common noun is a general name for a person, place, thing, or idea. It doesn't specify a particular entity and isn't capitalized unless it starts a sentence. In this case, 'car' refers to a vehicle but doesn't specify which one.
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