Which screw is used when great pressure needs to be applied?

The screw that is used when great pressure needs to be applied is the lag screw. Lag screws are heavy-duty fasteners designed for heavy applications where strong, secure connections are required, such as in wood construction. Their large, coarse threads allow them to bite deeply into materials, providing excellent holding power. They are often used in structural applications like securing beams or frames where the joint must withstand significant load and stress.

In contrast, the other options such as junction blocks, levels, and long nose pliers do not serve the same purpose as fasteners. Junction blocks are used for connecting electrical circuits, levels are tools for checking horizontal or vertical alignment, and long nose pliers are used for gripping and manipulating objects rather than fastening them together under pressure. Thus, lag screws are specifically designed for applications that necessitate high strength and pressure, making them the correct choice in this case.