Answer:
C. 50%
Explanation:
One of the grapefruit plant is fully white, while the other is heterozygous red:
fully white: bb (If it was BB, then the heterozygous would be white, as it would take on the dominant trait).
heterozygous red: Bb (Red is the dominant trait, denoted by B).
Create the Punnett square:
[tex]\left[\begin{array}{ccc}&b&b\\B&Bb&Bb\\b&bb&bb\end{array}\right][/tex]
As such, the fully white (bb) takes up 50% of the Punnett squre offsprings, which makes your answer C.50%
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