From the docs - http://docs.pylonsproject.org/projects/pyramid/en/1.3-branch/glossary.html#term-multidict :
multidict An ordered dictionary that can have multiple values for each key. Adds the methods getall, getone, mixed, add and dict_of_lists to the normal dictionary interface. See Multidict and pyramid.interfaces.IMultiDict.
So just call:
request.params.getall('c')