What is the problem called in which power and heat constraints prevent all circuits on a chip from running at full performance at the same time?

Answer: B. Dark silicon

With the end of Dennard scaling, power per transistor no longer drops enough even as transistor counts increase. Running all circuits on a chip at once would exceed the allowable heat, so some must be idled. This is called dark silicon, and it is part of the background for designs that use specialized circuits for different tasks.