How can you make sure the corners of a l-shaped building are square?
a. use a level or transit to take a reading at each corner of the layout.
b. divide the area into rectangles and check the diagonal measurement of each.
c. use the 3-4-5 rule.
d. use a plumb bob at each corner of the layout.

  C.  Use the 3-4-5 rule.

Explanation:

Many contractors will make use of the 3-4-5 rule for laying out square corners. I've seen a house foundation laid out that way, and the contractor building my deck used that rule to set the support beams.

In checking my deck contractor's work, I made measurements of the sides and diagonals of the deck and used a triangle solver to compute the angles at the corners. I found his 3-4-5 layout was off by about 1/4 degree, or about 3/4 inch over the width of the deck. (One diagonal is about 0.5% longer than the other.) To be fair, the deck is probably more square than the house to which it is attached.

These days, there are laser instruments that can be used to define square corners. They work well indoors, where lighting is reduced and contrast with the laser beam is better, and they work better on surfaces that show a light beam well.

The laser instruments I've looked at have an accuracy of about 1" in 100', about 0.05°, so maybe 1/5 the error of a 3-4-5 layout with a tape measure.

Alternate choice for the mathematically inclined

  B. Divide the area into rectangles and check the diagonal measurement of each.

Comments on the other choices

A level or transit is useful for defining "level", as opposed to "square." Likewise, a plumb bob is good for "vertical". A transit usually is capable of angle measurements with high accuracy (about 1/12 degree), so could be used to lay out square corners. The wording here associates a transit with a level, so suggests it is used for measuring vertical height, rather than corner angles.

Diagonal measurements alone can tell you a rectangle is out of square, but additional measurements are needed to tell what corrections are required. For an L-shaped building, the rectangles involved would need to be very carefully defined and measured. Just doing that could be problematic.