What tool is used to align CMUs with the line by tapping the top of the block?

The tool used to align concrete masonry units (CMUs) by tapping the top of the block is a trowel. A trowel, particularly a masonry trowel, features a flat blade that is useful not only for spreading mortar but also for adjusting the position of blocks during installation. When aligning CMUs, a contractor can gently tap the top of the block with the handle of the trowel to ensure it is level and properly set in line with adjacent blocks.

While other tools like a hammer or sledgehammer may be used in masonry for different purposes, they are less suitable for fine adjustments needed during the alignment process. A level is essential for checking horizontal and vertical alignment, but it does not actually make adjustments; it simply measures them. Thus, the trowel is the most appropriate tool specifically for the task of aligning CMUs by tapping the top of the blocks.