Draft A gets the purpose right. A bloom filter is a probabilistic structure for testing set membership, and A names the property that defines it: no false negatives. It meets the one-sentence limit. It doesn't mention false positives or the space savings, but it is still accurate and useful. Draft B also fits in one sentence, but "a thing for data" is so vague it says nothing about what a bloom filter is for, so it fails the task.

**WINNER: A**