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