Here the author wants to stress on the point that for a child parentage matters more than the environment of upbringing.

option D clearly emphasized on this claiming that from a tomato plant only tomato is expected as fruit not any other vegetables.None other option makes this more clear.Hence D

IMO - D also
agree with @liferocks

Thanks for clarification. I got it.

Author give more emphasis on genetic traits.

why can't it be C?
even C says the same thing that you will get tomatoes........

ansumania wrote:why can't it be C?
even C says the same thing that you will get tomatoes........

Option C gives more importance to environment and other factors than to genetics, for child's personality, which reverse of what auther is saying. This is why option C is wrong.

I do agree that there is emphasis on genetic makeup more than on environmental factors.
Both in C and D it is clear that if you plant tomatoes you get the a yield of tomotoes . Should one rule out C cos nowhere in the paragraph is it mentioned that the environmental factors can have a detrimental effect. So to conclude that the tomotoes will not come out proper is unjustified.

D makes no such guesses.