Let's analyze all the statements and see why they are false or true.
A. FALSE
The tallest plant in variety B is just 18 tall, while the variety A we have 21.
B. FALSE
We do have plants in A that have the same height as B.
C. FALSE
The standard deviation measure how far it's from the mean, the variety B has a 2.4 standard deviation, which means that the height can be more distant from the mean than in variety A.
D. True
Justified by C. Variety A has a 1.2 standard deviation, which means it's more likely to be closer to the mean