What does "assured us victory" mean?

[Nikita]

English Idioms and Expressions, Intermediate level

ESL/EFL Test #5 "As happy as the day is long", question 6

That last comment of yours won the day because nobody could possibly argue with you after that.

(a) assured us victory
(b) assured us the answer
(c) assured us progress
(d) assured us agreement

English Idioms and Expressions, Intermediate level

ESL/EFL Test #5 "As happy as the day is long", answer 6

That last comment of yours assured us victory because nobody could possibly argue with you after that.

Correct answer: (a) assured us victory

Your answer was: incorrect
That last comment of yours assured us agreement because nobody could possibly argue with you after that.
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What does "assured us victory" mean?

Nikita

[Mister Micawber]

.
Guaranteed that we would win.
.
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Native English teacher at Mister Micawber's