This is ----- bike I bought yesterday.

Correct answer: the

Explanation: Use 'the' because the noun 'bike' is uniquely identified by the clause 'I bought yesterday'. That clause tells us which specific bike is meant, so the definite article is required.

• Why not 'a': 'a' is an indefinite article for a non-specific singular noun; it would imply any bike, not the particular one bought yesterday.

• Why not 'an': 'an' is used before words beginning with a vowel sound; 'bike' begins with a consonant sound, so 'an' is not appropriate.

• Why not no article: Singular countable nouns usually need an article or determiner; omitting the article here would be ungrammatical.

Correct sentence: This is the bike I bought yesterday.