Which of the Following Is Correct About a Consumer's Optimization

Which of the following is correct about a consumer's optimization problem:
a.
In order for a consumer to not be optimizing at a corner solution,it is necessary for us to assume that all goods are essential.
b.
In order for a consumer to not be optimizing at a corner solution,it is sufficient for us to assume that all goods are essential.
c.
In order for a consumer to not be optimizing at a corner solution,it is necessary and sufficient for us to assume that all goods are essential.
d.
None of the above.