In the above mentioned sentence, the most appropriate word to be filled in the blank is Option A, “Lately.”
The sentence describes a habit that has been perceived in the recent past, which forms the earlier part of the sentence, and the literal meaning of the word ‘lately’ matches with the same. Though it includes the word late, the word lately means something regarding the recent past, that has been started not much earlier than the present situation.
The other options, “late” and “later”, mean different points of time which do not match with the blank, while “latter” would mean the next one. It will not apply in the sentence as only one party is involved, hence Option A is the right answer.