Answer:
B). It was a hot, humid afternoon when we decided to head for the beach.
Step-by-step explanation:
In the given question, option B reflects the sentence with a correct comma usage as it joins the two coordinating adjectives together that modifies/qualifies the same noun 'afternoon'. The other options are incorrect as they place the comma inappropriately.
In option A, the comma precedes the noun 'afternoon' that is grammatically incorrect while in options C and D, the coma precede the conjunction 'when' which is also grammatically incorrect as according to the rules a comma precedes a conjunction only when two independent clauses are joined together which is not the case here(an independent clause is being followed by a dependent clause here). Thus, option B is the correct answer.