What is the difference between 'stand on him' and 'stand by him'?

[Reign18]

Business Idiom in English, Intermediate level

ESL/EFL Test #131 "Ways of approving", question 9

They all promised they would stand ......... him when he put forward the idea of a strike.

(a) on
(b) to
(c) by
(d) through

Business Idiom in English, Intermediate level

ESL/EFL Test #131 "Ways of approving", answer 9

They all promised they would stand by him when he put forward the idea of a strike.

Correct answer: (c) by

Your answer was: incorrect
They all promised they would stand on him when he put forward the idea of a strike.
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in lesson 31 " ways of approving" whats the diffence of 'stand by him' and 'stand on him'

need help thnx;-)

[Kitosdad]

" Stand on him ", means you can rely on this person to do what he says he will do. He is reliable.

" Stand by him ", means you would take his side in any confrontation. You are on HIS side.
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Englishman living in Germany.