Which province of northern Italy was part of Caesar's proconsular command?

The province of northern Italy included in Caesar’s proconsular command was Cisalpine Gaul.

Cisalpine Gaul, or Gallia Cisalpina, lay on the Italian side of the Alps and included the Po Valley and surrounding regions. Its name literally means “Gaul on this side of the Alps,” from the Roman perspective. Although geographically part of northern Italy, Rome traditionally treated it as a separate provincial and military zone during the late Republic.

In 59 BC, the Lex Vatinia assigned Caesar the proconsular command of Cisalpine Gaul and Illyricum for five years. The Senate later added Transalpine Gaul—roughly the Roman province in southern France—after its governor died. This command gave Caesar troops, money, and a secure base near Italy, helping launch the Gallic Wars in 58 BC.

Cisalpine Gaul is easily confused with Transalpine Gaul. “Cisalpine” means on the near, Italian side of the Alps; “Transalpine” means beyond the Alps. Caesar governed both during his wider Gallic command, but the question’s reference to northern Italy specifically points to Cisalpine Gaul.