Answer: a.

Explanation:
Use a comma and “which” with nonrestrictive clauses. Nonrestrictive clauses add information but aren’t essential to the meaning of the sentence. Hard drive erasers all work by overwriting data, so the addition isn’t required — it just explains how those products work.

Use “that” without a comma with restrictive clauses. A restrictive clause is essential for meaning. For example:

The most common method of wiping a hard drive is using a product that overwrites data to render it unreadable.

The restrictive clause is “that overwrites data to render it unreadable” and we need it to understand the sentence because if we just say “The most common method of wiping a hard drive is using a product,” that doesn’t give us much information.

Richard Nordquist provides more explanation of restrictive and nonrestrictive clauses.

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