“Whether or not” vs. “whether”
"I was unsure whether to pity him."
"I was unsure whether to pity him or not."
Is it redundant? Yes. Is it unacceptable in speech or writing? No. But ultimately, the question is if it is redundant, or not, which it is.
Just to be clear, because my redundancy joke might confuse:
It is redundant, though it might be useful for emphasis or clarity.
I used to write "whether or not" a lot, and I personally make a deliberate effort to remove the "or not"s.
In your example, "I am unsure whether to pity him", it is clear that there are two options: you might pity him, or you might not. Adding "or not" doesn't add any information. But it can be useful when you are trying to build dramatic tension into a sentence. Consider, "I don't like Bob, but he has a right to live, too." Versus, "I don't like Bob, but he may have a right to live ... or not."
AHED defines whether:
Looking at it yet another way:
So the OP's sentence becomes:
It's perfectly clear that not (alternative B) is acceptable, by sense 2 or sense 3 of the dictionary definition.