_________fracture occurs when there is a straight break right across the bone.

The correct option is Transverse.

To understand why this is the correct answer, let's break down the definitions and characteristics of the different types of bone fractures and injuries mentioned in the options:
1. Transverse Fracture: This type of fracture occurs when a bone is broken in a straight line directly across its long axis. The break is perpendicular (at a 90-degree angle) to the length of the bone. This matches the definition of a "straight break right across the bone" perfectly.
2. Greenstick Fracture: This is an incomplete fracture in which the bone bends and cracks, but does not break completely into separate pieces. This type of fracture is most common in children because their bones are softer and more flexible than adult bones.
3. Strain: A strain is not a bone fracture at all. It is an injury that occurs when a muscle or tendon (the tissue connecting muscle to bone) is overstretched or torn.
4. Comminuted Fracture: This refers to a severe fracture where the bone breaks or splinters into three or more pieces.

Therefore, a transverse fracture is the term that specifically describes a straight break perpendicular to the shaft of the bone.