Combination of Resistors question from NEET 1996 (AIPMT)

Answer: (a)

1. B and C are open ends, so the end resistors there carry no current.
2. From the junction after $R_1$ to the junction before $R_4$: path $R_2 + R_3 = 20\ \Omega$ and path $R_6 + R_5 = 20\ \Omega$ in parallel $= 10\ \Omega$.
3. $R_{AD} = R_1 + 10 + R_4 = 10 + 10 + 10 = 30\ \Omega$
4. So the answer is (a) $30\ \Omega$.