Why not C?

I agree D does not show why blood pressure should fluctuate when they sign. There is no reasoning in the argument that shows that signing might mean communicating like speech does for the blood pressure to rise. If it is used as an analogy to support or prove that the argument is right then why not A. In A scientific data is shown that inspite of taking the medication, blood pressure fluctuates when speaking showing that speech has an effect on blood pressure

Hi, A and C are wrong because it is not relevant if someone has high or low blood pressure in absolute terms. We are trying to prove that the stress of communicating leads to fluctuations. Someone with high, normal, or low blood pressure can experience fluctuations. D, however, directly supports the idea that the stress of communication leads to fluctuation. Signing is arguably a form of speech, but undeniably a form of communication.