Acertain trait is represented by a capital letter r or a lowercase letter r. each parent is homozygous for the trait. which are the possible genotype combinations for their offspring? check all that apply. rr rr rr rrr r pr pr

The question is not giving a clear trait of the parents. I will explain it assuming the parent is homozygous dominant (RR) and homozygous recessive (rr).

Because the parent is homozygous, the allele that will be inherited would have only 1 possible ways. It will be like this:
parent 1: RR ---> R
parent 2: rr >r
When both allele combined, you will get 100% Rr trait.

If the question asking if the parents are any combinations of the homozygous trait, then:
1 parent homozygous dominant, 1 parent homozygous recessive: 100% Rr
both parent homozygous dominant: 100% RR
both parent homozygous recessive: 100% rr

True, the standard unit for volume is liters which can be represented by either a lower case "l" or a capital "l"