By standing by his side and pushing him on.
1. to(prep) Chicago (obj)
2. by(prep) car(obj)
3. By(prep) afternoon(obj)
4. for(prep) lunch(obj)
5. near(prep) river(obj)
6. on(prep) water(obj)
7. for(prep) trip(obj)
8. of(prep) Chicago(obj)
9. at(prep) motel(obj)
A preposition tells where one noun is in relation to another noun. It is always followed by a noun, which is the object of a preposition. One silly way to remember most prepositions is to think about a squirrel and a tree. A squirrel can go (through, on, under, in, off, to, by...) the tree. There are a few prepositions that just need to be remembered such as for.
The answer to your question would be A
