Idom: tie the knot

[Deya]

Test No. incompl/advan-30 "Letter Writing (2)", question 8

This brings me to another point: why aren't you and Sara tying the .........?

(a) noose
(b) rope
(c) knot
(d) string

Test No. incompl/advan-30 "Letter Writing (2)", answer 8

This brings me to another point: why aren't you and Sara tying the knot?

Correct answer: (c) knot

Your answer was: incorrect
This brings me to another point: why aren't you and Sara tying the noose?
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What does 'tying the knot' means here? I can't understand the sentence.

[Alan]

Hi Deya,

This is an oldfashioned expression for getting married.

Alan
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English as a Second Language
You can read my ESL story Present Simple