For potentiometer the voltage source is providing constant current in the circuit and if \\\\(R\\\\) and \\\\(X\\\\) are connected in series it will form Wheatstone's network so the balancing condition is\\\\(\\dfrac {X}{R}= \\dfrac{70}{60}\\\\)\\\\( X = \\dfrac {7\ imes 12}{6} = 14\\Omega\\\\)equivalent resistance of \\\\(R\\\\) and \\\\(X\\\\) in series\\\\(=R+X= 26 \\;ohms\\\\).Hence, correct answer is option B.