Answer: The correct answer is -
B. when at least one allele contains instructions for tall.
As per the given information in the question, tall is a dominant characteristic.
Dominant characteristic (represented by capital letter allele such as 'T') is the one that always masks the expression of recessive characteristic (such as dwarf, that is represented by small letter allele 't').
Tall can possibly have two genotype that are Tt (heterozygous dominant) and TT (homozygous dominant).
It means that presence of single dominant allele (T) in the genotype of the trait would result into the dominant character that is tall. In other words, if one allele carries instruction for tall, it will definitely give tall character.
Thus, option B) is the right answer.
