Among the following. the number of elements showing only one non-zero oxidation state is:
$\mathrm{O}. \mathrm{Cl}, \mathrm{F}. \mathrm{N}, \mathrm{P}. \mathrm{Sn}. \mathrm{Tl}. \mathrm{Na}. \mathrm{Ti}$

Sodium (+1) and fluorine (-1) show only one non zero oxidation state.
The oxidation states of oxygen are -1, -2 and +2.
The oxidation states of chlorine range from -1 to +7.
The oxidation states of nitrogen and phosphorus range from -3 to +5.
The oxidation states of tin range from +2 to +4.
The oxidation states of thallium are +1 and +3. +3 oxidation state is rare but does exist.
The oxidation states of titanium are +2, +3, +4 respectively.
Among the following $\mathrm{O}. \mathrm{Cl}, \mathrm{F}. \mathrm{N}, \mathrm{P}. \mathrm{Sn}. \mathrm{Tl}. \mathrm{Na}. \mathrm{Ti}$, the number elements showing only one non-zero oxidation state is 2.