I have a problem about reading section of Kaplan book

In the conventional classroom, most lessons are aimed at the middle level of ability. Thus, some students are rushed along much faster than [color=red]optimal, or faster than necessary for satisfactory results.

the word [color=red]optimal in the passage is closest meaning to
a)usual
b)possible
c)plentiful
d)profitable

I think usual is correct but the book choose profitable.

please explain for me this question.

Thank in advance.

TOEFL listening lectures: A university lecture by a professor of Art History

Hi Moravid, in the following sentence, “In the conventional classroom, most lessons are aimed at the middle level of ability. Thus, some students are rushed along much faster than is (you forgot to write down “is”)optimal, or faster than necessary for satisfactory results”, the book chose the right choice. The correct answer is profitable because here the meaning of profitable is “worthwhile or useful or gainful” and optimal means"the most desirable" and profitable should be chosen among the others.
I hope I answered your question. Let’s see what other students comment.

Hello, Babo, thank you for your help, after seeing your comment i checked my book again but it does not “is” in that statement and i did not forget it.

cheers

Optimal = the most favorable conditions for success, so as Babo says, ‘profitable’ which means ‘most beneficial or useful’ is the correct choice here.
It’s possible to add ‘is’ to the sentence where Babo has indicated without changing the meaning, but its usage is optional there.

Well I checked this too in my book Kaplan 4th edition. There was “is” in that sentence. Never mind you got your answer, that’s what was needed:).

Thank you dear Babo for your help.

My pleasure:)