Question 4
In activity 04 a second 20 N block is placed on top of the first. What happens to the limiting friction between the bottom block and the table?
The right answer is the one marked above.
It increases, because the normal reaction increases. The table now supports 40 N instead of 20 N, so R doubles, and limiting friction rises with R.
"The surfaces have not changed" is true but incomplete — the nature of the surfaces is only one of the two factors, and the other one has just changed.