What does "unknown quantity" means?

[Frida_st]

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #558 "English Slang Idioms (196)", question 2

He always became nervous when reporting to a new boss. It was hard to start over again with an unknown ......... especially one he didn't know how to please. He couldn't even rely on the past to derive the boss' expectations.

(a) entirety
(b) quantity
(c) persona
(d) ability

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #558 "English Slang Idioms (196)", answer 2

He always became nervous when reporting to a new boss. It was hard to start over again with an unknown quantity especially one he didn't know how to please. He couldn't even rely on the past to derive the boss' expectations.

Correct answer: (b) quantity

Your answer was: incorrect
He always became nervous when reporting to a new boss. It was hard to start over again with an unknown persona especially one he didn't know how to please. He couldn't even rely on the past to derive the boss' expectations.
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Please, explain what " unknown quantity" means? does it refer to a new boss and why??

Frida