Second and third floor studs must be:

The correct choice is 2" x 4" for the studs used on the second and third floors in typical residential construction. This dimension is widely recognized in the building industry for framing purposes due to its balance of strength and weight.

Studs of this size are designed to support the load of walls, ceilings, and roofs above, as well as resist lateral forces. They provide adequate strength for the majority of residential applications while allowing for standard insulation installation and drywall attachment. Using a 2" x 4" stud is compliant with various building codes and practices, making them the preferred choice in standard construction.

Larger sizes such as 3" x 6" or 3" x 4" would typically be overkill for most residential framing needs. They could be more suitable for specific structural applications that demand additional load-bearing capacity, but they add unnecessary weight and cost. Similarly, 2" x 5" studs are not a common dimension used in framing and do not comply with standard practices for second and third-story construction. This makes the 2" x 4" the most practical and standard choice for framing in this context.