Explanation

(d)
Given constraints are
$x_{1} + 2x_{2} \leq 2000,\ x_{1} + x_{2} \leq 1500,\ x_{2} \leq 600$ and $x_{1} \geq 0$
The feasible region is $OACDEO$
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At $O(0,\ 0),\ z = 0 + 0 = 0$
At $A\ (1500,\ 0),\ z = 1500 + 0 = 1500$
At $C(1000,\ 500),\ z = 1000 + 500 = 1500$
At $D(800,\ 600),\ z = 800 + 600 = 1400$
At $E(0,\ 600),\ z = 0 + 600 = 600$
Here, $z$ is maximum on the segment $AC$
Hence, there are infinite optimal solutions