What is the maximum resistance which can be made using five resistors each of 1/5 Ω?

The maximum resistance which can be obtained from a group of resistors is always the series combination of those resistors.

Here, R_eq = (1/5) + (1/5) + (1/5) + (1/5) + (1/5) = 5/5 = 1Ω

NOTE: For series combination, R_eq=R₁+R₂+…..+R_n (For n resistors)