What does let on mean?

[Farsa]

English Language Tests, Intermediate level

ESL/EFL Test #195 "Phrasal Verbs (L)", question 1

Jeff felt as if he had been ......... when he lost his job.

(a) let on
(b) let down
(c) let over

English Language Tests, Intermediate level

ESL/EFL Test #195 "Phrasal Verbs (L)", answer 1

Jeff felt as if he had been let down when he lost his job.

Correct answer: (b) let down
_________________________

What does let on mean?

[Yankee]

.
Have a look at definition 12 here.