Question

how can the two chromosomes that make up a homologous pair differ?
they can contain different alleles for the same trait.
they can contain genes that code for different traits.
they can contain the same genes in different orders.
they can contain a different number of genes.

Explanation:

Homologous chromosomes are pairs of chromosomes that have the same length, centromere position, and gene loci. They carry genes for the same traits. However, the alleles (different forms of a gene) for those traits can differ. For example, one chromosome in a homologous pair may carry the allele for brown eye color and the other may carry the allele for blue eye color.
Genes on homologous chromosomes code for the same traits (so the option about different traits is wrong). Homologous chromosomes have the same gene order (so the option about same genes in different orders is wrong) and the same number of genes (so the option about different number of genes is wrong).

Answer:

They can contain different alleles for the same trait.