Shipwrecks ....

can anyone explain this? Thanks.

I will Try my best.

Shipwrecks are more likely to be found undisturbed at ........... than in .............

The above construction uses the correct idiom More ... Than. The other constructions like "More ..... as opposed to" and "More ..... Instead of " are incorrect.

Thus, our contenders are first two options. Of these two, the first one can be rejected because of the use of Ambiguous pronoun "they", and of the use of non-parallel construction "which exposes archeological remains to turbulence and (which) makes them accessible to anyone .... ".

The option (b) is more concise and clear.

Any other thoughts welcome !

More likely so than is needed at the start
hence CDE ruled out

A is wrong
because they are is not needed
they refers to what?
anyone => they is wrong

Also
"Shallow waters, which exposes.." is incorrect
Shallow water does not expose


Hope this helps