Answer:
D. Things that are difficult to understand or explain.
Step-by-step explanation:
The word "mystery" refers to the thing(s) difficult to understand or comprehend, or even explain. It suggests things that are hard to be understood or even explained to others, anything that mystifies or is secretive and unable to be understood.
As used in the given sentence, the word "mysteries" refers to the things that are hard to understand, the things in the sea that appeals to her yet are incomprehensible.
Thus, the correct answer is option D.