'Expose' is a verb which means to put somebody/something or yourself in a situation that could be difficult or dangerous, so an adverb is required in place of 'harmful' at (A) in order to qualify/modify the verb. 'Intentionally' is the only correct adverb. Here, it conveys that the researchers cannot put patients in danger purposefully. 'Compounds' is a noun so an adjective is required before that to qualify it; 'harmful' is the correct adjective which means 'dangerous'.
Therefore, the right order is BAC.
Hence, option (b) is correct.
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