As atomic number increases within group 15 on the periodic table, atomic radius
(1) decreases, only
(2) increases, only
(3) decreases, then increases
(4) increases, then decreases

Answer is: (2) increases, only.

The atomic radius of a chemical element is a measure of the size of its atom.

The atomic radius varies with increasing atomic number, but usually increases because of increasing of number of electrons.

Nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), bismuth (Bi) and moscovium (Mc) are in group 15 of Periodic table. They have same number of valence electrons (five).

Nitrogen has smallest atomic number and smallest atomic radius.

if you mean hydrogen and oxygen then water is the product

answer : one can design an experiment using a liquid and an additive that can be added to the liquid and observe the chemical changes in the reaction species.

one would need a beaker which is filled with a known amount of the liquid in which thermometer is placed to measure the temperature changes; after addition of the additive to the liquid solution one can note down the temperature change and find whether the reaction is endothermic or exothermic in nature. by subtracting the initial temperature value with the final temperature one can find the temperature changes.

various combinations of liquid and additive can be used. liquid in this case can be a known quantity of water and additives can be epsom salt or without epsom salt the experiment can be conducted to notice the changes. water and ice, hydrogen peroxide and dry yeast, vinegar and baking soda,etc.