Correct Answer: familiar

Solution : The correct choice is the first option.

Explanation:
"Familiar" means recognised or known from past experience. In the context of the sentence, it suggests that the person recognises the voice, possibly from previous encounters or familiarity.

The meanings of the other options are as follows:

• Similar means resembling but not identical.
• Customary means usual or habitual.
• Routine means ordinary or commonplace.

Therefore, the complete sentence would be: "While I was walking along the beach, I heard a familiar voice."