She was waiting for _____ cab. 

Correct option is B

 • “Cab” is a countable singular noun.

• We use “a” before singular countable nouns when referring to something general.

• Correct sentence:
She was waiting for a cab.

Meanings of other options:
• few – used for plural nouns
• any – used in negative/interrogative
• the – used for specific noun

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