The main thing is to be consistent.
For example, " Clinical trials and Drug Development" looks disorganised, because you have capitalised 'Drug Development' but not 'trials'.
There are few universally accepted rules about capitalisation, and Britain and the US have different conventions : the British think that Americans 'capitalise' too much, while Americans think that the British don't 'capitalize' enough.
Of the three options you've given for the 2nd degree, "Clinical trials and drug development" is the best. Or you could also follow the more American convention and capitalise the content words: '"Clinical Trials and Drug Development". This is a title, so I think that looks fine.
Likewise '"Impact of Environment on Health" is fine, as you've capitalised the main content words but not the little grammar words. This is a good compromise between under- and over-capitalisation.