The correct option is (a) 6 V
The best I can explain: For finding the Thevenin Equivalent circuit across A-B we remove the 5 Ω resistor.
Then, I = \(\frac{10+50}{15}\) = 4 A
VOC = 50 – (10×4) =10 V
And REQ = \(\frac{10×5}{10+5} = \frac{10}{3}\) Ω
Current I1 = \(\frac{10}{10/3+5} = \frac{6}{5}\)
Hence, VAB = \(\frac{6}{5 × 5}\) = 6 V.