Use appropriate articles where necessary-
When I was ________ child, I used to be very shy.
a. a
b. an
c. the
d. no article

Hint: The student should read the sentence carefully and recall the usage of the three articles in the English language. Then the option which contains the right for this sentence should be selected.

Complete answer:
A definite article is a determiner (as `the' in English) that indicates the specificity of reference. An indefinite article is a determiner (as `a' or `some' in English) that indicates nonspecific reference.

In the given question, we have to find out which of the options indicates a nonspecific reference (as child is not a specific reference here) and is used for a word that starts with a consonant sound (c- child).
Let us thus analyze the options given to us in this question -
Option (a.), 'a', refers to an indefinite article used before words starting with consonant sounds for a nonspecific reference.
Therefore, option (a.) is correct as we needed an answer that indicates a nonspecific reference (as child is not a specific reference here) and is used for a word that starts with a consonant sound (c- child).
Option (b.), ‘an', refers to an indefinite article used before words starting with vowel sounds for a nonspecific reference. Therefore, option (b.) is incorrect as we need an answer that is used for a word that starts with a consonant sound (c- child).
Option (c.), ‘the', refers to a definite article used before words that have a specific reference.
Therefore, option (c.) is incorrect as we need an answer that indicates a nonspecific reference.
Option (d.), ‘no article', is incorrect as we need an answer that indicates a nonspecific reference (as child is not a specific reference here) and is used for a word that starts with a consonant sound (c- child).

Note:
 In this question, it will be helpful for the student to recall the concept of articles. An article is (grammar) a determiner that may indicate the specificity of reference of a noun phrase. There are two types - indefinite and definite.