Which one of the following words can be used as a substitute

Please wait...
THANKU FOR BEING A PART OF OUR JOURNEY TO BRING "REVOLUTION IN EDUCATION"
We Genuinely APPRECIATE your PATIENCE

Coming Soon

M: +0.84/-0.27

Which one of the following words can be used as a substitute for the phrase, “That which cannot be taken by force”?

[UPSC EPFO 2017]

Impossible

Improbable

Impeccable

Impregnable