What is the constraint for node $2$ in the following maximal flow problem?

Node $1$ connects to Nodes $2$ and $3.$ Node $2$ connects to Nodes $3$ and $4.$ Node $3$ connects to Node $4.$ The Network has a return arc connecting

Node $4$ to Node $1.$ The coefficients for Nodes $1?4$ are: $+0,+0,+0,$ and $+0,$ respectively. The arc between Nodes $1$ and $2$ is $4.$ The arc

between Nodes $1$ and $3$ is $5.$ The arc between Nodes $2$ and $3$ is $2.$ The arc between Nodes $2$ and $4$ is $3.$ The arc between Nodes $3$ and $4$ is $6.$

a$.$

X$12<=4$

b$.$

X$12+$ X$13$ X$23=0$

c$.$

X$12$ X$23$ X$24=0$

d$.$

X$12+$ X$23+$ X$24=0$