Hi,
Making use of contemporary diaries and letters,
Florey’s complexity is brilliantly revealed by Lax.

The sentence means that Lax reveals Florey’s complexity by making use of contemporary diaries and letters.

Now let’s spot the errors in the sentence. This sentence has just one subject-verb, meaning we have just one clause here. This sentence starts with a verb-ing modifier “making”. This modifier denotes an action. Hence, the doer of that action should be the subject of the following main clause. However, Florey’s complexity is the subject here. So “making” is modifying Florey’s complexity. This modification is incorrect. It is Lax who has made use of the diaries and letters. Hence, the subject of the IC should be Lax.
POE:A. Florey’s complexity is brilliantly revealed by Lax:
Incorrect for the reason stated above.
B. Florey is brilliantly shown in all his complexity by Lax:
Incorrect. Same modification error as in choice A.
C. Lax brilliantly reveals Florey’s complexity:
Correct.D. Lax brilliantly revealed the complexity of Florey:
Incorrect. This choice uses the incorrect verb tense.
E. Lax’s study has brilliantly revealed the complexity of Florey:
Incorrect. Verb-ing “making” is incorrectly modifying “Lax’s study”.
Hope this helps.
Thanks.
Shraddha