Which is correct and most complete?

a. Starting with standard screening using fractional factorials is natural because interactions are modeled.

b. They could reasonably use a single, standard Box Behnken array with four factors.

c. A reasonable recommendation is to perform two Box Behnken DOEs, one for each level of the categorical factor.

d. There is no way to use any RSM method since they have a categorical factor.

e. All of the above are correct.

f. All of the above are correct except

(a) and (e).