Here are the best-fitting words for each gap:

1. **d. before**
2. **b. in**
3. **c. contribute**
4. **a. with**
5. **a. When**
6. **d. volunteers**

So, the complete sentences will read:

"Doing some form of voluntary work has (1) **been more popular with British people. Over 20 million people were engaged (2) **in** voluntary activities in 2013. Volunteering means giving up time to do work of (3) **contribute** to the community. It can be based in the UK or overseas. Volunteering can take many forms, from working with children with (4) **difficulties, in an animal hospital, or planting trees. (5) **When** London won its bid to host the 2012 Olympics, up to 70,000 (6) **volunteers** were needed to help ensure the games were a success."