To prepare a good temporary mount of the petunia leaf peel showing many stomata, the student has to get the peel from the :
(1) tip of the leaf
(2) upper surface of the leaf
(3) lower surface of the leaf
(4) point of attachement of leaf to its petiole

To prepare a good temporary mount, the student should take the peel from the lower surface of the leaf.
Stomata are structures which help in gaseous exchange and are usually scattered on the lower surfaces of leaf.

Hence, the correct answer is option 3.