c
As the two cells oppose each other hence, the effective emf in closed circuit is \(15-10=5 \,\mathrm{V}\) and net resistance is \(1+\) \(0.6=1.6\, \Omega\) (because in the closed circuit the internal resistance of two cells are in series.

Current in the circuit,

\(I=\frac{\text { effective emf }}{\text { total resistance }}=\frac{5}{1.6} \,A\)

The potential difference across voltmeter will be same as the terminal voltage of either cell. 

Since the current is drawn from the cell of \(15\, \mathrm{V}\)

\(\therefore \quad V_{1}=E_{1}-I r_{1}\)

\(=15-\frac{5}{1.6} \times 0.6=13.1 \,\mathrm{V}\)