The correct answer is: D. Vertical line
A bed joint is a horizontal joint between masonry units. A perpend is a vertical joint between masonry units. A lintel is a horizontal structural member that spans and supports the weight of a wall above an opening, such as a door or window. A vertical line is a line that extends from top to bottom without any deviation.
In the context of the question, the alignment of a cross joint along the plumb line is a vertical line. This is because a plumb line is a vertical line that is used to ensure that a structure is vertical. The alignment of a cross joint along the plumb line ensures that the masonry units are laid in a vertical position. This is important for the structural integrity of the masonry wall.
Bed block, perpend, and lintel are all terms that are used in masonry construction. However, they do not refer to the alignment of a cross joint along the plumb line.