In the periodic table, the period number indicates a. the number of dots shown on the lewis dot diagrams for elements in that period. b. the number of valence electrons for elements in that period. c. the reactivity levels for elements in that period. d. the number of the outermost energy level for elements in that period.

B. the number of valence electrons for elements in that period.

This is how the period number indicates the number of valence electrons:

Period      number of valence         elements
                 electrons

1              1                                     Li, Na, K, Rb, Cs, Fr
2              2                                     Be, Mg, Ca, Sr, Ba, Ra
13            3                                     B, Al, Ga, In, Tl, Nh
14            4                                     C, Si, Ge, Sn, Pb, Fl
15            5                                     N, P, As, Sb, Bi, Mc
16            6                                     O, S, Se, Te, Po, Lv
17            7                                     F, Cl, Br, I, At, Ts
18            8                                     He, Ne, Ar, Kr, Xe, Rn, Og

As you see, for the representative groups the last digit of the period number is the number of valence electrons.

It was ernest rutherford