The sentence that includes a comma splice is "The days lengthened, summer was here".
What is a comma splice?
A comma splice refers to a comma mistake that generally happens when two independent sentences are joined using a comma. The comma splice is when there is no joining word between two independent sentences.
The sentence "The days lengthened, summer was here" has two independent sentences that are joined using a comma. They could be joined using a joining word.
Hence the correct option is B.
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