We need to choose between the present perfect and past simple, and sometimes use the negative form. This depends on whether the action is completed at a specific past time or has relevance to the present.
Step-by-step solution:
B: No, thanks. I haven't had anything. (The action of having dinner is in the past, but the current moment is 'eight o'clock', so present perfect is appropriate for a recently completed or not-yet-completed action).
I have had three lectures today and I still have two more later this afternoon. (The lectures happened today, which is still ongoing, so present perfect is used).
It was so hot today that I wore shorts and a T-shirt. (The heat and wearing clothes are in the past, a specific event, so past simple is used).
We have spent £200 on food this month and there's another week to go before I get paid. (The spending happened 'this month', which is still ongoing, so present perfect is used).
B: No, I didn't oversleep this morning because my alarm clock didn't go off, so I need to work late. (The event of oversleeping is a specific past event that did not happen, hence past simple negative).
I haven't read much of the report yet, but I have to finish it by the weekend. (The reading is an action that started in the past and continues up to the present, hence present perfect negative).