According to the NCERT Table 4.11 detailing the oxidation states of Actinium and Actinoids:

1. Actinium (Ac, Z=89): Shows only the +3 oxidation state.
2. Thorium (Th, Z=90): Predominantly shows the +4 oxidation state (NCERT lists only +4 for Th).
3. Protactinium (Pa, Z=91): Exhibits +3, +4, and +5 oxidation states.
4. Uranium (U, Z=92): Exhibits +3, +4, +5, and +6 oxidation states.

Thus, Actinium (Ac) is the only element among the options that exhibits exclusively the +3 oxidation state .