A groundbreaking neuro-psychological research was conducted on epilepsy patients, whom brains' neural activity is regularly monitored, and have discovered that memories of past events are stored in the same neurons in the brain which were active during the original experience.
A. whom brains' neural activity is regularly monitored, and have discovered that
B. whom brains' neural activity is regularly monitored, and has discovered that
C. whose brains' neural activity is regularly monitored, and have discovered that
D. using data from the regular monitoring of their neural activity to establish that
E. using data from the regularly monitoring of their neural activity to establish

A - This answer choice is grammatically incorrect. Whom is a relative pronoun describing humans who are the object of the relative clause. This relative clause has no object. It consists of a subject (the brains' neural activity) and a verb (is monitored) only. The relationship between the epilepsy patients and this relative clause is one of possession, since it is their brains' activity which is being monitored. The relative pronoun used to describe a relationship of possession is whose.
This answer choice includes another grammatical error. The plural verb have does not agree with the singular subject research.
B - Although this answer choice corrects the second of the original mistakes mentioned above, it repeats the first and is still grammatically incorrect.
C - This answer choice, on the other hand, corrects the first of the original mistakes, while repeating the other and thus it is still grammatically incorrect. The plural verb have does not agree with the singular subject research.
D - Bingo!
This answer choice is the only grammatically correct answer choice.
E - The -ly suffix identifies regularly as an adverb. Adverbs are used to describe the manner or degree in which an action is performed. Therefore, their main function is to modify verbs, while they can also modify adjectives or other adverbs.

Now let's look at the word which the adverb regularly modifies in this answer choice - monitoring. The word monitoring can be clearly identified as a noun in this case thanks to the article which precedes it - the. Since an adverb (regularly) cannot modify a noun, the phrase the regularly monitoring is grammatically incorrect.