Explanation:

- Though: This option introduces a contrast between the fact that it was snowing and the decision to continue driving. It implies that despite the adverse conditions, the decision was still made. This fits the context perfectly.

- However: This word is typically used to introduce a contrast or a contradiction, but more often at the start of a new sentence or clause, rather than directly connecting two actions in the same sentence.

- If: This word is used to introduce a condition, suggesting that one event is dependent on another, which doesn't fit the given context.

- Unless: This option is used to introduce an exception, implying that one thing will happen except under certain conditions, which doesn't match the structure of the sentence.