Minimum value of current - op-amp?
Voltage at non-inverting input is 3.75V and therefore voltage at inverting input is also 3.75V.
Current through top 30k resistor is 3.75/30k = 125uA and this will be the current through R irrespective of its value.
With R = 0 Ohms, current through 10k load is 3.75/10k = 375uA
Therefore with R = 0 Ohms, Io = 375uA + 125uA = 500uA which is the minimum value of Io.
Any increase in R above 0 Ohms increases the voltage across 10k load and therefore increases Io above its minimum value of 500uA.