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A Gerund is a word ending with a -ing and functions as a noun therefore your answer is C) The exercise we all avoid is running. Running ends with a -ing and its the only one that functions as a noun. Hope I helped! Thanks for asking us here at Brainly, Please feel free to ask any more questions!
Answer: C) Running
Explanation: a gerund is a type o verbal noun, it means that is a verbal form (that ends in -ing) that acts like a verb in terms of accepting modifiers such as adverbs, but the whole clause (sometimes just one word), acts like a noun.
In the provided sentences the only one that shows an -ing form acting like a noun, is the option C: The exercise we all avoid is running, notice that we can change the word "running" to be the subject of the sentence: "Running, is the exercise we all avoid". In the first and second options, the -ing form acts as participles, and in the last one as an adjective.