==> The total mass resting on the table is (5 kg + 3 kg) = 8 kg.
==> The total weight of that mass is (8 kg) x (9.8 m/s) = 78.4 newtons
==> The boxes are stacked. So the table doesn't know if the weight on it is coming from one box, 2 boxes, 3 boxes, or 100 boxes in a stack. The table only knows that there is a downward force of 78.4 newtons on it.
==> The table stands in a Physics classroom, and it soaks up everything it hears there. It knows that every action produces an equal and opposite reaction, and that forces always occur in pairs.
Ever since the day it was only a pile of lumber out behind the hardware store in the rain, the table has known that in order to maintain the good reputation of tables all over the world, it must resist the weight of anything placed upon it with an identical upward force. This is the normal thing for all good tables to do, up to the ultimate structural limit of their materials and construction, and it is known as the "normal force".
So the table in your question provides a normal force of 78.4 newtons. (d)
