In airplane construction, titanium is most commonly used in which parts?

Titanium is preferred in specific parts of airplane construction because of its high strength-to-weight ratio, excellent corrosion resistance, and ability to withstand high temperatures. In the context of airplane engines, titanium is often utilized in components such as fan blades, engine casings, and other vital parts that need to endure extreme conditions while maintaining structural integrity and performance. The material's properties also make it suitable for aircraft frames, where rigidity and lightweight performance are critical.

The other options include parts of the airplane that generally rely on materials better suited to their operational environments and requirements, such as aluminum or composites that are typically used in the wings and fuselage or non-load-bearing areas. While titanium is indeed used in landing gear due to its strength, its predominant usage is concentrated in engine and frame components due to the specific demands of these areas.