(b) The image below shows part of the surface of a leaf.
0.1 mm x 0.1 mm
The length and width of this piece of leaf surface are both 0.1 mm

Calculate the number of stomata per mm of this leaf surface.

Explanation:

You didn't attach a picture so i'll just explain how to find the number of stomatas/mm^2.

first you find the surface by multiplying the length by the width

0.1*0.1=0.01mm^2

now you count the numer of stomatas in the picture

in0.01mm^2 you havex stomatas

in 1mm you have y stomatas

y=100x

well, that's easy; the answer's c, "it reduces contamination of the water and substrate of the estuary."