The recent photographs of the giant squid are remarkable because they show these enormous living creatures as moving around in their natural environment, whereas previous pictures have been of only dead animals.

A. because they show these living creatures as moving around in their
B. in that they show this most enormous of living creatures moving around in its
C. in that they show this enormous living creature moving around in its
D. because these enormous living creatures are shown to be moving around in their
E. because they show this enormous living creature moving around in a

This question is related to Subject-verb agreement.
giant Squid is Singular.
so option A is out --> these is plural.
option B is out --> enormous of living creatures : enormous is Adjective. so 'of' is not required.
option D is out --> these is plural.

Now out of C and E --> difference is
moving around in its or moving around in a ?

Can someone explain me..how to eliminate the wrong option now ?

Here is how I will work out -

1. 5 options are there, seeing the similarity I will divide the 5 options in 2 parts - one starting with "because" and other with "in that".
2. You can filter/reduce the options by choosing 1 part. I will chose the part starting with "in that". Reason: the one starting with "because" is wrong, there will be double because. e.g., The recent photographs of the giant squid are remarkable because because they show.......................
3. Part with "in that" - 2 options are there -
B. in that they show this most enormous of living creatures moving around in its
C. in that they show this enormous living creature moving around in its
Here "they" is refering to the photographs and not the squid.
How?
Explaination: The recent photographs of the giant squid are remarkable ... Take out "of the gaint squid" (modifier) from the sentence (for discussion sake), so sentence becomes - The recent photographs are remarkable ........

4. Now see option B, "most enormous" is extra info and not mentioned in the sentence. "most enormous" and "enormous" are different, rt?
5. So you are left with only option C.