The correct option is
D Really
! When did you apply for this job?
Here, we have to choose the best response for the given sentence. If someone gives us a good news about them or shares something nice, we should always respond positively, politely and express happiness. Let's have at look at the options.
Option A - 'Why didn't you tell me?' is an incorrect and a rude response, because the person who shares his joys with us, expects us to be happy about them. Hence, option A is incorrect.
Option B - 'I don't believe you' is a sarcastic comment and can't be used as a response to someone's new job. Hence, option B is incorrect.
Option C - 'You are lying' is an incorrect response as it shows that the person doesn't want to accept the news. It doesn't express the listener's emotions clearly. Thus option C is incorrect.
Option D - 'Really!' is used to express excitement and happiness, hence this is the correct expression. 'When did you apply for this job?' is also a correct response as it shows that the listener is interested in knowing more. Hence, option A is correct.