1. We need the element that shows only one oxidation state other than its elemental form.

   • Elemental form means oxidation state $0$.
   • So we want the element which, in compounds, shows essentially only one oxidation state.

2. Check each option:

   A: Nickel ($\mathrm{Ni}$)
   Nickel commonly shows: $+2, +3$ and sometimes other states in complexes.
   So it does not show only one oxidation state.

   B: Titanium ($\mathrm{Ti}$)
   Titanium shows multiple oxidation states: $+2, +3, +4$ Hence, not correct.

   C: Cobalt ($\mathrm{Co}$)
   Cobalt commonly shows: $+2, +3$ So it also has more than one oxidation state.

   D: Scandium ($\mathrm{Sc}$)
   Scandium predominantly shows only: $+3$ This is because after losing three electrons, it attains the stable noble-gas configuration.

3. Therefore, the element showing only one oxidation state other than $0$ is: $\boxed{\mathrm{Scandium}}$

4. Comparison with stored correct answer:

   • Derived answer: $\boxed{D}$
   • Stored correct answer: $\boxed{D}$

They match.