Why is "clock didn't go off"?

[Olena]

English Language Tests, Intermediate level

ESL/EFL Test #164 "Idioms with the phrasal verb go", question 4

Rosie overslept because her alarm clock didn't ......... this morning.

(a) go off
(b) go over
(c) go on
(d) go back

English Language Tests, Intermediate level

ESL/EFL Test #164 "Idioms with the phrasal verb go", answer 4

Rosie overslept because her alarm clock didn't go off this morning.

Correct answer: (a) go off

Your answer was: incorrect
Rosie overslept because her alarm clock didn't go on this morning.
_________________________

Why is "go off"?

Olena

[Conchita]

If an alarm clock doesn't go off, it doesn't ring.