What is the name of a wood member that is graded by its wider dimension, measuring 2-4 inches thick and at least 6 inches wide?

The correct answer is "planks." In the context of wood grading, planks are defined as wood members that are thicker (typically 2-4 inches) and wider (at least 6 inches) than dimensional lumber. This wider dimension is significant when it comes to applications like flooring, paneling, or other situations where a broader surface area is necessary.

Understanding the properties of planks is essential for construction and design. They are used when a sturdier or broader piece of wood is required, often to support larger loads or to cover greater spans.

The other options pertain to different classifications of wood. Joists are typically smaller, structural members used to support floors or ceilings and are graded based on their depth and span capability. Beams are larger structural supports but usually do not fall within the specified dimensions of being 2-4 inches thick and at least 6 inches wide—they are often much larger. Lumber is a general term referring to wood that has been processed into beams, planks, and other forms, and doesn't specifically describe the wider dimensions referenced in the question.