Answer:
It can be both, because the definition of blithe is “showing a casual and cheerful indifference considered to be callous or improper.” And that is negative, but the other definition is “happy or joyous.” Which is positive
Explanation:
The correct answer is Carol has a cottage near the shore.
This supporting detail has nothing to do with her study of marine life. She could have this cottage because she inherited it, or she just likes the place - it doesn't really contribute to her love or study of marine life. The other options clearly support why she is studying marine life, whereas the one with the cottage does not.
The last sentence is correct, because the other ones either had periods in the middle or too many commas in them.