Could somebody please tell me with word is correct?

Which word or preposition is correct please?

Can your 401k be used for/as a down payment on/for a house?

Gambling could put your family in jeopardy/could jeopardize your family.

The country came out of/left communism in...

My car caught fire/caught on fire.

What shape does/do two equilateral triangles make?

Do you want the light dimmer/weaker or stronger/brighter?

Thank you very much