most rapidly

[azz]

a. A lot of people finished the same assignment. John did it fastest.
b. A lot of people finished the same assignment. John did it the fastest.

================================

c. A lot of people finished the same assignment. John did it most rapidly.
d. A lot of people finished the same assignment. John did it the most rapidly.

Are all of the above grammatically correct?

I don't know if 'John did it most rapidly.' is ambiguous or not.

I think (a) and (b) mean the same. It seems to me that the 'the' is optional there.

But could (c) be used instead of (d) or does it only mean that he did it very rapidly?

Many thanks.

[Alan]

Yes, all are grammatically possible, but (a) and (c) are improbable in this context, since they compare John's speed at the time in question with other times (as opposed to a comparison at the same time with other people, which is what is meant here), e.g.

John always worked fastest after a good breakfast.