Which 1755 British expedition was sent to capture Fort Duquesne?

The 1755 British expedition sent to capture Fort Duquesne was the Braddock Expedition.

General Edward Braddock led the campaign during the opening phase of the French and Indian War. His force included British regulars, colonial troops, artillery, and support personnel. The objective was to seize Fort Duquesne at the Forks of the Ohio, the strategic confluence now occupied by Pittsburgh, and weaken France’s position in the region.

The army departed Fort Cumberland in Maryland in May 1755 and struggled to move heavy guns and supplies across the Appalachian Mountains. George Washington accompanied Braddock as an aide-de-camp and provided local knowledge, but Braddock’s force was defeated near the Monongahela River on July 9. Braddock was mortally wounded, and the survivors retreated.

The Forbes Expedition was a separate British campaign that successfully captured the French fort in 1758. The Kittanning Expedition targeted a Native settlement in 1756, while Amherst’s major North American command came later in the war. Thus the Braddock Expedition is the specific canonical answer for 1755.