No, the word "proposal" is not an adverb.The word "proposal" is a noun.
Suggestion is another word for proposal answer: engagement?
Proposal letters are of many types, for example, business proposal letter, sponsorship or marriage proposal letter. For a better insight you can surf the web. I found a website for you, see if it is useful.
The word is spelled proposal, just as you spelled it. Propose is the root word.
The link below has an example of business proposal letter, hope it will be helpful.
The following source is an example of proposal letter for sponsorship. Hope it works !
Daily is a good word for a 5 letter word for everyday.
small 5 letter word
The 5 letter word for equal is 'alike'.
loose is a 5 letter word for not taut.
Inter is a 5-letter word for bury.
what 5 letter word begins with s- a living thing-second 5 letter word ending in d