Positive cooperativity would most likely be observed when a protein is:

a$.$

a single polypeptide with a Kd $=1010.$

b$.$

a single polypeptide with three ligand$-$binding sites of increasing Kd values.

c$.$

comprised of two identical subunits, each of which binds ligand.

d$.$

comprised of two ligand$-$binding sites where the Kd for the second site decreases after the first ligand binds.

e$.$

comprised of subunits A and B$,$ where ligand binding to A facilitates ligand binding to B$.$