An independent clause must contain a
subject
subject and a predicate
predicate
subordinate clause

The answer is B. subject and a predicate

B) Subject and a predicate.

Explanation: an independent clause is a phrase that has complete meaning on its own (it is the antonym of dependent or subordinate clause, which is a phrase that doesn't have complete meaning when it is by itself). An independent clause should always have a subject (the person or thing that performs the action of the verb) and a predicate (that can be just a verb, or also have complements and modifiers). So the correct answer is option B.

the following key ideas correctly express the key sentence.

restate key idea: most people can learn to swim in ten short lessons.

restate key idea: the majority of water-accident victims require mouth-to-mouth breathing.

restate key idea: an empty, tightly closed gallon jug will support a tired swimmer.  

restate key idea: first aid is to prevent accidents as well as assist in rescue.

explanation: the key idea needs to have the same topic as the key sentence (drowning and safety).