Answer: Option 'c' is correct.
Step-by-step explanation:
Since we have given that

We need to find the number of units in order to minimize cost.
We first derivative w.r.t. x,

For critical points:

Now, we will check whether it is minimum or not.
We will find second derivative .

So, it will yield minimum cost.
Minimum cost would be

Hence, At 150 units, minimum cost = $4000
Therefore, Option 'c' is correct.