(i) Establish a relationship to determine the equivalent resistance $R$ of a combination of three resistors having resistances $R_1$, $R_2$ and $R_3$ connected in parallel.

(ii) Three resistors are connected in an electrical circuit. Calculate the resistance between A and B.

The question asks us to establish a relationship to determine the equivalent resistance R of a combination of three resistors having resistances R1, R2, and R3 connected in parallel. We can use the formula: 1/R = 1/R1 + 1/R2 + 1/R3. In this case, the resistors are connected in parallel, so we can add their reciprocals to find the equivalent resistance. The correct answer is (a) 4Ω.