The adopted use of trained fighter horses, which some

Now, we have to look at the options very clearly in order to ensure we pick the best of the options.

Option A - INCORRECT.
This option presents us with no meaningful contribution to the subject matter because it doesn't bring up any other intuition aside just copying from the question.

Option B - INCORRECT
This option meant to explain the same line of meaning which isn't clearly what we need. We need to speak with one voice although using different languages. Instead of the sentences here we should have had something like ''which some historians thought to have occurred'', take note of the past tense use of ''occur'' because it happened in the past.

In Option C - INCORRECT.
Here, using a continuous form of the verb ''adopt'' is wrong. You can only continue with what's happening in the present terms not what has happened in the past in linguistic sense though. Clearly not correct.

In Option D - CORRECT.
This statement is specific and affirmative. The verb ''adopt'' has been nominalised here, i.e it has become a noun, which is good for a logical change of how the sentence has been written though still retaining the meaning. The corrections made to Option Bcould be noticed here, ''occur'' has become ''occurred''.

In Option E - INCORRECT.
This statement is though a correct option but it's not easily comprehensible. Logic needed to explain it. It can't be easily explained to a layman.