Parveen thank you for your kind words. I still have a few licenses to the GMATFix App to give away and it looks like you are eligible; if you find my videos helpful, you will have access to tons of helpful material within the App.
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Ok let's get back to the question at hand. Focusing on the sentence core, let's examine choices that use "whose" and "which"
B)
[Narwhals] seek sanctuary from whales, and
their migrations which follow the rythm
This is a sentence fragment; we would expect a verb after migrations to make the 2nd part match the first ("they seek sanctuary"). Pronoun "which" interferes with the parallel structure (compare to C).
D)
[Narwhals] seek sanctuary from whales, and
whose migrations following the rythm
E)
[Narwhals] seek sanctuary from whales, and
whose migrations follow the rythm
As D & E are written, "they seek...and whose migrations follow(ing)" is not a parallel construct. In both of these choices, the 2nd part is far from being parallel to the first. For "and whose" to be right, we would expect the first part of the construct to also be introduced with a relative pronoun as in the example below:
eg: Narwhals,
which can be called whales of the ice, and
whose migrations follow a rythm, seek sanctuary from killer whales.
By contrast, consider the right answer's parallel construct:
C)
[Narwhals] seek sanctuary from whales, and their annual migrations follow the rythm.
Hope that helped,
-Patrick