What proceeds the comma will modify "The ocean". Looking at the answer choices, one finds that they are identical for the first half ("a beautiful deep blue hue in the light and...") and that they all make use of the conjunction 'and'. What proceeds 'and' needs to modify "The ocean" as if "a beautiful deep blue hue in the light and..." were not there, while still making sense within the sentence.
Altering the answer choices a little to make things easier:
(A) The ocean, in stretching as far as the eye could see
This choice makes no sense and does not work with the first part of the non-essential clause.
(B) The ocean, a stretch as far as the eye could see
This choice makes no sense. Using 'a' in front of 'stretch' makes it seem like calling this ocean an ocean is a stretch.
(C) The ocean, stretching as far as the eye could see
Perfect. Modifies 'the ocean' to illustrate its vastness, while linking back nicely to the first half of the non-essential clause.
(D) The ocean, stretches as far as the eye could see
One issue here is the clash of tenses. 'Stretches' is present tense while 'could see' is in the past. Overall this choice just does not work.
(E) The ocean, being stretched as far as the eye could see
Makes very little sense. The ocean cannot be stretched, rather it stretches as far as the eye can see.
Answer C