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I think c
[dictionary.reference.com...]
so I guess all you really do is login, plain and simple
B and C can both be correct depending on context.
If you want to use a verb to describe the action of "logging in" you would use "Log in".
If you want to use a noun to describe the place of exercising said verb, it may very well be a "Login" prompt. The collected name for the form, along with required authentication credentials, should always be referred to with the grouping name "login".