Hey There!
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Answer:
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
It is the physical appearance of the trait, whether the plant has Long stem or short stem.

A genotype refers to the genetic characteristics of an organism's trait, if it is heterozygous or h o m o-zygous, recessive or dominant. This question is basically based on the Genotype, asking for the heterozygous plants which is a genotypic characteristic.
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Alleles:
Allele Long stem A is dominant over short stem a
Genotype of Parent Pea Plants
Pea Plant 1 has genotype Aa
Pea Plant 2 has genotype aa
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The question asks for how many heterozygous plants are present in the punnet square of pea plant 1 and pea plant 2.
Heterozygous Plants
Heterozygous Plants means those plants which have two different alleles that means One dominant allele (i.e. A here) and One recessive allele (i.e. a here). These Organisms inherit a different version from each parent. In a heterozygous genotype, the dominant allele overrules the recessive one.
If we look at the punnet square given in the attachment, the Box 1 and 3 have genotype Aa which is a HETEROZYGOUS DOMINANT PLANT while box 2 and 4 have a genotype aa which is a HOMO-ZYGOUS RECESSIVE PLANT thus the answer is Box 1 and 3
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Best Regards
'Borz'