33. A man who can roll his tongue and a woman who cannot roll her tongue have a son who can roll his tongue (R = can roll tongue; r = can't roll tongue). The son is curious about whether his father is homozygous or heterozygous for the tongue-rolling trait. Which of the following facts would allow him to know?
a. His father's mother cannot roll her tongue.
b. His paternal grandfather and his paternal grandmother can both roll their tongues.
c. His sister is a tongue roller.
d. His own daughter cannot roll her tongue.

Question

33.	A man who can roll his tongue and a woman who cannot roll her tongue have a son who can roll his tongue (R = can roll tongue; r = can't roll tongue). The son is curious about whether his father is homozygous or heterozygous for the tongue-rolling trait. Which of the following facts would allow him to know? 
a.	His father's mother cannot roll her tongue.
b.	His paternal grandfather and his paternal grandmother can both roll their tongues.
c.	His sister is a tongue roller.
d.	His own daughter cannot roll her tongue.

alphadxg · Answer

Well you C wouldn't work that wouldn't give us anything, D - same reasoning.

A - wouldn't work because we already know about the father.

So you are left with one answer that makes total sense.