Answer:
A. Amy is correct. Kevin did not line up the decimals correctly.
Explanation:
Let the grams of water added = x
[tex]\begin{gathered} 2.3+x=4.21 \\ \implies x=4.21-2.30 \\ x=1.91\text{ grams} \end{gathered}[/tex]From the workings shown, we see that Amy is correct as Kevin did not line up the decimals correctly.
The correct choice is A.