What is something parents can do to encourage volunteering?
a. talk to their children about their interest
b. volunteer themselves
c. suggest a trial run with an volunteer position
d. all of the above

I would go for All of the above, to answer the question: What is something that parents can do to encourage volunteering?

Explanation:

The reason for all the other options being the correct fit for the question is that all of them are actions by parents that can help encourage  a child. Talking to them about their interests will help parents point their children in the best direction, and it may also help them to advice children on those services that may best suit them. Also, it is true what they say that you learn better from example, and seeing a parent volunteer their time may increase the interest of a child in the activity. Also, the last one, suggesting a trial run will encourage the person to try out different options, without any pressure, while finding out what he/she is truly interested in doing. This is why the last option encompasses all the possible things that parents could do to encourage volunteering in their children.