Nouns ending in -tion are feminine.
The suffix forms abstract nouns of action or state.
Is proposition masculine or feminine?
laproposition
The rule behind it
99.8% of -tion nouns are feminine
In context
Tu aurais dû refuser une proposition aussi injuste.
You should have refused such an unfair proposal.
J'ai été forcé d'accepter sa proposition.
I was forced to accept her proposal.
Je doute que la nouvelle proposition sera acceptée.
I doubt the new proposal will be accepted.