What is the maximum resistance which can be made using three resistors each of resistance 1/3 Ω?

We have been given the resistance of each resistor = 1/3 Ω

Number of resistors = 3

We know that equivalent resistance is maximum when resistors are in a series combination. 

The equivalent resistance is given by,  

R_s = R₁ + R₂ + R₃ 

⇒R_s = 1/3 + 1/3 + 1/3

⇒R_s = 3/3

Hence, the correct option is (d) 1 Ω.