A false assumption is typically based on a premise that may not be true without evidence or can be logically questionable given the context.
In this case:
A. "My brother is taller than my mother" - This could be true or false; it’s not necessarily a false assumption. B. "My baby brother will grow taller than my sister" - This is uncertain but not necessarily false; growth patterns can vary. C. "My brother is over six feet tall" - This can be true or false, depending on actual measurements. D. "My older brother is taller than my sister" - This could also be true or false.
However, B is the best fit for a false assumption in that it presumes a future outcome (growth height) without evidence. We cannot definitively assume a baby brother will outgrow a sister.
So the answer is B: "My baby brother will grow taller than my sister."