Why use "pull it off" in this sentence?

English Language Tests, Intermediate level

ESL/EFL Test #197 [color=blue]“Phrasal Verbs (P to R)”, question 6

After weeks of planning the escape, Sue finally started to believe they could .

(a) pull it off
(b) pull it forward
© pull it up

English Language Tests, Intermediate level

ESL/EFL Test #197 [color=blue]“Phrasal Verbs (P to R)”, answer 6

After weeks of planning the escape, Sue finally started to believe they could pull it off.

Correct answer: (a) pull it off

Your answer was: [color=green]correct
[size=200]_________________________[/size]

Why use “pull it off” in this sentence?

That they would be able to escape successfully.