**Solution:**

The correct answer is: at the intersection of at least two constraints.

Definition:

A linear programming problem (LP) is an optimization problem for which:

1. We attempt to maximize (or minimize) a linear function of the decision variables. (objective function)

2. The values of the decision variables must satisfy a set of constraints, each of which must be a linear inequality or linear equality.

3. A sign restriction on each variable.

For each variable xi the sign restriction can either say

(a) xi ≥ 0,

(b) xi ≤ 0,

(c) xi unrestricted (urs)

Hope this might help you.