Which combining form is used in reference to the "sole of the foot"?

The combining form that refers to the "sole of the foot" is indeed "pod/o." In medical terminology, "pod/o" is derived from the Greek word "pous," which means "foot." This combining form is commonly used in various medical terms related to the foot, such as "podiatry," which is the branch of medicine concerned with the diagnosis and treatment of foot disorders.

The other options do not relate to the foot. "Pleura" refers to the membrane surrounding the lungs, while "pentan/o" relates to five-carbon molecules, often used in chemistry and biochemistry. "Phren/o" pertains to the diaphragm or mind in medical terminology. Thus, the choice "pod/o" stands out as the correct combining form for the context of the question.