What does "talk him down" mean?

[Henna]

English Language Tests, Intermediate level

ESL/EFL Test #706 "English Slang Idioms (448)", question 3

One of the scariest moments of my career as a police officer was when an idiot said he was going to shoot a two-year-old if he wasn't given a million dollars. Luckily, we were finally able to talk him .......... He dropped the gun and came away peacefully.

(a) down
(b) back
(c) out
(d) away

English Language Tests, Intermediate level

ESL/EFL Test #706 "English Slang Idioms (448)", answer 3

One of the scariest moments of my career as a police officer was when an idiot said he was going to shoot a two-year-old if he wasn't given a million dollars. Luckily, we were finally able to talk him down. He dropped the gun and came away peacefully.

Correct answer: (a) down
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I need the explantion of the word

[Mister Micawber]

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Which word?
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Native English teacher at Mister Micawber's