A block of mass M is kept on a friction floor of static friction μ. The body is acted by a horizontal force F horizontally but the body does not move. Now an another f act on the body vertically downwards, what will happen :

(a) frictional force on the block change
(b) normal reaction on the block change
(c) limiting frictional force changes
(d) body starts moving

b and c

Normal reaction will change as N = mg + f

Limiting friction is $ f_l = \mu \times N = \mu \times (mg+f)$ will also change.