In which group of the periodic table do most of the elements exhibit both positive and negative oxidation states?
a. 17
b. 2
c. 12
d. 7

The answer is:

A. 17

The explanation:

first we have to define the oxidation state:

It is the loss of the electrons in the atom of the component , or the amount of the oxidation or the degree of the oxidation of an atom.

and the group 17 consist of Halogens:

1- Fluorine : with atomic number 9 , The oxidation state of it is always -1 but in its diatomic case the oxidation state is zero.

2- chlorine: Cl with atomic number 17 , The oxidation states of it are multiple oxidation states for example -1, +1, 3, 5, and 7.

3- Bromine: Br with atomic number 35 , The oxidation states of it are -1, +1, 3, 4 and 5.

4- Iodine: I with atomic number 53 , The oxidation states of it  are oxidation states -1, +1, 5 and 7

5- Astatine: At with atomic number 85, The oxidation states of it are -1, +1, 3, 5 and 7

So this group (17) exhibit both positive and negative oxidation states.