The elements present in horizontal rows of a periodic table have same:
(A) Atomic mass
(B) Molecules
(C) Valence shell
(D) Valence electrons

Hint: Periods in the periodic table are the horizontal rows in which elements are present. The vertical columns present in the periodic table are called groups. The elements present in the same group have similar chemical properties.

Complete step by step answer:
We have to find which property of the elements is not going to change in horizontal rows of the periodic table.
Coming to the given options, option A, atomic mass. As we are moving from left to right in the periodic table atomic mass means the molecular weight of the elements is going to increase because the atomic number and number of protons and number of neutrons are going to increase. So, option A is wrong.
Coming to option B, Molecules. As we are moving from left to right in the periodic table the elements lose the capability of formation of molecules because metallic character is going to decrease from left to right in the periodic table. so, option B is also wrong.
Coming to option C, Valence shell. As we are moving from left to right in the periodic table the elements present in the same period have the same valence shell. Means the new electrons are going to add into the same valence shell in the same period. So, option C is correct.
Coming to option D, Valence electrons. As we are moving from left to right in the periodic table the number of valence electrons is going to increase because new electrons are going to add to the same valence shell. So, the option D is wrong.
Therefore the elements present in horizontal rows of a periodic table have the same Valence shell.

So, the correct option is C.

Note: There are seven periods in the periodic table. Valence shell remains same across the period but down to group valence shell changes. Means across periods the electrons are going to add to the same valence shell but in case of groups the electrons are going to fill in different valence shells.