I'm not sure what (c) is about. Is there a space, which can be filled in, after 'to hold the property'? If so, I suggest that it's just there to allow for 'non-standard' forms of ownership. (e.g. where the couple own shares in the house which are of different sizes. See below)
Joint tenants don't have a transferable share in the house. i.e. one person can't leave their share in the house in their will. If the owners are joint tenants, when one partner dies the whole of the house automatically becomes the property of the surviving partner.
Tenants in common, however, each own a specified share in the house (which, if they want, they can leave to someone other than their partner in their will). The shares can be equal (which is obviously what box (b) on the form is for) or they can be unequal. (I'm guessing that, if there's a blank space to be filled in, that's what box (c) is for).
Source:
http://www.crossmans.co.uk/services/conveyanci ng/co_ownership.asp
Chris