You look tired. Why don’t you _______________ and have a good rest. A. call it a day B. call its name C. call on D. call off I know that the answer is “A. call it a day” – an expression which means stop working for the present. However, I wonder why we couldn’t choose D. call off which also means “stop”. Thanks in advance.
The simple answer is that 'call off' needs an object, but the sentence offers none.

I would like to tell you something about the choosing the best of the alternatives.

It is always better to go with the one which sounds natural immediately. In questions of this type you always get one option sometimes even two which are almost correct.