Look at the straight line that cuts at (0,50) and (50,0).
The equation of the line is x+y=50.
The feasible region (shaded region) includes the line and the part above it.
Hence, the constraint it x+y≥50.
Now, look at the dashed line which cuts at (0,90) and (90,0).
The dashed line has the equation x+y=90.
If it is dashed, the line is not included in the feasible region i.e (0,90) and (90,0) are not part of the feasible region.
The shaded region is below this line.
Hence, the constraint is x+y<90.
Hence, option 4 is the answer.