For me the best option is "a". See the following:
-Now we're looking for at least ten volunteers who can wrap the presents (everybody can wrap a present, what you want is people doing that job. The use of the relative here is not necessary, because a volunteer is precisely somebody who can do something)
-Now we're looking for at least ten volunteers in order to wrap the presents (it doesn`t sound familiar to me. If you asked me, I would omit using this phrase. There you are not specifying that the volunteers will be the ones who are going to wrap the presents)
-Now we're looking for at least ten volunteers now we need to wrap the presents (IMPOSSIBLE!)
-Now we're looking for at least ten volunteers for the wrapping of the presents (it`s not used, so it would be grammatically wrong. Perhaps you can here it from a native speaker, but it is not correct)
The only way of using the word "volunteer" is with the "to + inf.verb" following it. Otherwise, you are wrong. If you don`t believe me, look it up in a dictionary and you will find manifold examples with "volunteer to do sth."
Here are some examples from Oxford Advanced Learner`s Compass:
-Schools need volunteers to help children to read.
-Are there any volunteers to help clear up?