The correct answer is D. Betty's eyes stung from the salt water, but she was having too much fun to get out of the ocean.
Explanation
When it is necessary to indicate ownership of a name in English, it is necessary to put an apostrophe and an "s" to indicate that the person, place, or thing is the owner of the noun that appears after its name. In this sentence the apostrophe is being used in the word Betty's, that is to say, that it is referring to some possession of Betty, in this case, it refers to her eyes because "eyes" is the word following the word with an apostrophe. Therefore, the correct answer is D. Betty's eyes stung from the salt water, but she was having too much fun to get out of the ocean.
