The given sentence requires an adverb before the word 'hot'. We could have used option A 'hardly', but the later part of the sentence says 'Let it cool down a little bit', which means that the tea is very hot.
Hence we can't say that the tea was hardly hot. Thus option A is incorrect.
We could have used option B 'scarcely hot' which means 'hardly/barely hot', but we need an adverb that can replace the meaning of 'very hot'. Thus option B is incorrect.
Option C is incorrect, as 'Hard' here is an adjective whereas we need an adverb.
Option D is correct, the correct sentence is - The tea is too hot; I cannot drink. Let it cool down a bit. 'Too hot' means very hot, thus it describes the degree of how hot the tea is.