To complete the sentence, you would typically use the word "runny" instead of a synonym with a more negative connotation. However, if presented with options like "runny" versus "liquid," "liquid" has a more neutral tone, while "runny" suggests a less desirable state for dough.
So the completed sentence would be:
"After he added too much milk to the batter, Shawn's dough became too runny."
If "runny" is among your options, that would be the choice with the more negative connotation.