Good question! This is an important lesson to always pay attention to quantifier words. Answer choice A says, "Most people who have tried typing with non-QWERTY keyboards have typed significantly more quickly using those keyboards than they usually have done using QWERTY keyboards."
Do we have support to conclude that "most" people who tried the non-QWERTY keyboard had this result? No. Let's consider the specific results of the study. "Experiments have shown that keyboard configurations more efficient than QWERTY can double typing speed..."
We know that this result "can" happen, but we don't know how often. This is why we cannot conclude "most." Also, the study mentions more efficient keyboard layouts, while answer choice A simply says "non-QWERTY."
If you are going to choose an answer with a quantifier, make sure that it is supported.