Why use "pull it off" in this sentence?

[Mickey]

English Language Tests, Intermediate level

ESL/EFL Test #197 "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
(c) pull it up

English Language Tests, Intermediate level

ESL/EFL Test #197 "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: correct
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Why use "pull it off" in this sentence?

[Kitosdad]

That they would be able to escape successfully.
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