The correct option is D ill at ease
To feel at home means to be/feel/look relaxed and comfortable in a particular place or situation. The opposite of feel at home is ill at ease, or uncomfortable or embarrassed. Hence, Option D is correct. Among the rest of the options, unwelcome means (of a guest or new arrival) not gladly received, unhappy means not happy, unpleasant means causing discomfort, unhappiness, or revulsion. These options do not express this antonym, hence incorrect.