Question about Thevenin theorem
This doesn't need anything other than joining the identical sources together and treating the combination as one single 24 volt source feeding via 0.8||0.9.

@Andyaka I understand your point but the question stated that thevenin
theorem has to be applied in this question. So how should I apply the
theorem to show my understanding of it?


Turn the left-hand 24 volt source and its respective series resistor (0.8 ohms) into a current source in parallel with its equivalent resistance.
Do the same for the right-hand 24 volt source and its respective series resistor (0.9 ohms) into a current source in parallel with its equivalent resistance.
Combine the current sources into one current source
Combine the 0.9 ohm and 0.8 ohm into one resistor then...
reverse the process and determine the combined equivalent voltage source and the combined series resistance and then solve standard voltage divider equations to ascertain the voltage across \$R_0\$ based on what value \$R\$ is.

Can you take it from here?