Correct Answer: (a) 'Amateur' is a noun which refers to a person who takes part in an activity as a hobby or for pleasure and not as a job.
Solution:We generally use the indefinite article 'a' before the words starting with a consonant sound and article 'an' before words starting with a vowel sound. Similarly, in the given sentence, 'noun', 'person', 'hobby', 'Job' are some words that are starting with a consonant sound & 'activity' is the only word that starts with a vowel sound. Hence, 'Amateur' is a noun which refers to a person who takes part in an activity as a hobby or for pleasure and not as a job' is the most appropriate answer.