The correct answer is \(C\).

Given,

masses \(=2\,kg , 4\, kg\)

\(f =10\,N\)

So,

Force \((f)=(2+4) a\)

or,

\(a=\frac{f}{6}=\frac{10}{6} m / s ^2\)

Let the force of spring is \(T\)

so,

\(T=2 a=2 \times \frac{10}{6}\)

\(=\frac{10}{3}\,N\)

Thus, the spring force between the two blocks will be \(\frac{10}{3}\,N\)