Question

How can you help your neighbors reduce their unhealthy weight gain?

Answer 1

Modeling is always the best way to teach. So I would say B.

Answer 2

Model useful habits probably 
When you are eating at restaurants, you and them are eating and that is one source of weight gain.
Having discussions might shame them and make them embarrassed and not motivate them
Walking to their house should be normal if you are their neighbor as they should only be a few houses down

When you are modeling habits for them, them and their kids may pick up the habits and be healthier people. 
Hope this helps :)