A.
"However" can be an adverb, or it can be a conjunction.
As a conjunction it joins two clauses together, and it is found at the beginning of the second clause:
I would like to contribute to your cause; however, I don't have any spare cash.
Strunk and White advise against beginning a sentence with "however."
When "however" is used as an adverb, it is acceptable to put it at the beginning of a sentence:
However hard he tries, he won't succeed if he doesn't follow all the steps.