Meaning of call it a day

[Veki]

Test No. incompl/inter-38 "Save the Day", question 3

It's getting late now and I'm very tired so I think we'll ......... it a day.

(a) name
(b) describe
(c) call
(d) label

Test No. incompl/inter-38 "Save the Day", answer 3

It's getting late now and I'm very tired so I think we'll call it a day.

Correct answer: (c) call
_________________________

Again me:-)
I guess what is in question, but I can't be 100% sure, please explain what does mean " call it a day"?
Thank you

[Torsten]

If you call it a day you stop working and start relaxing.

TOEIC listening, photographs: Looking at the building plans