Which statement about phenotypic plasticity is false?
I'll try to answer but I'll encounter some difficulties: first because I don't know what you mean by "plasticity". Does it mean non-reversible developmental plasticity or reversible flexibity. Then, because, I don't really like the word adaptation! Anyway'll try...

a: Plasticity  is some kind of mechanism allowing organisms to adapt to various environment. If the environment is stable and under the assumption the plasticity is associated with a intrinsic (physiological for example) cost, then a population would tend to evolve so that they always display the most optimal phenotype. I would argue that this is under the assumption that we gave enough time for the right mutations happen and under the assumptions that such mutation might exist!

b I don't really understand this statement. Does it mean that between two generations (which mean in one generation time) an adaptation cannot vary.…in what terms? in frequency? Through time any trait vary because of selection, environment, genetic drift and mutations. And to what environment is the trait under consideration adapted for? Can we accept that the environment is changing from one generation to the other in this question? What does this question has to do with plasticity? I guess that when one says that a trait is an adaptation it means (among other things) that the population has no more variance for this trait. Therefore, only mutations would bring variations if there were no environmental change making the trait not adaptive anymore.

c: Well, yes. Plasticity is an adaptation to fluctuating environment. It allows to display a rather optimal phenotype in multiple different environments.

Answer d is wrong for sure. There is variation within a population due to genetic variance and non-adaptive environmental variance (and their interactions). Variance due to plasticity is just some kind of subset of the variance du to the genetic-environnment interaction

Hope this helps a bit!