You decide to designate the alleles of the four loci as either additive (contributing to fruit segment number and represented by a superscript "+") or non-additive (not contributing to fruit segment number and represented by a superscript "0"). Using this convention, choose the correct genotype for the two pure lines and the F1, and indicate how many additive alleles each genotype has.

The correct genotype of the  two pure lines and the F1 is:

A⁺A⁺B⁰B⁰C⁰C⁰D⁰D⁰   and A⁰A⁰B⁺B⁺C⁺C⁺D⁺D⁺

The number of additive alleles on each genotype are two and six respectively.

Explanation:

Locus( plural form . loci) are fixed point on a chromosome in which genes are located. These genes are specific genetic material or genotype.

Now;

If we decide to designate the allele of the four loci into either additive (⁺) or non-additive(⁰); we have the following :

Let's the allele of the four loci to be

A⁺/A⁰, B⁺/B⁰, C⁺/C⁰ and D⁺/D⁰

However, from the diagram below; we deduce that the correct genotype for the two pure lines and the F1 is as follows:

A⁺A⁺B⁰B⁰C⁰C⁰D⁰D⁰   and A⁰A⁰B⁺B⁺C⁺C⁺D⁺D⁺ and the number of additive alleles on each genotype are two and six respectively.

The cross between both F1 traits will yield an heterozygous individual for the offspring. i.e A⁺A⁰B⁺B⁰C⁺C⁰D⁺D⁰ with only four additive allele

the correct option is option a