(b) That's wrong. The threat in this case is of economic harm (financial damage to B's business) rather than physical harm. Such threats may constitute duress.
However, the courts distinguish carefully between illegitimate compulsion and behaviour falling within the accepted (often harsh) cut-and-thrust of normal business activity. The critical question is whether the threat was in some way unlawful. In the present case, the threat to wrongfully terminate the lease on a pretext is clearly unlawful and amounts to duress.
Suppose A had said: "If you don't agree to the increase, I won't renew your lease after it expires". Such a threat is not in itself unlawful and would not constitute duress.
Sometimes even a threat to do something that is lawful might constitute duress if, in the circumstances, making the threat amounts to unconscionable conduct, because unconscionable conduct is unlawful.