Anil is too fast a runner not to come first in the race.(Begin:
Anil is so...)

Answer:
Anil is so fast a runner that he could not come first in the race.

Why is the sentence written below not correct?

Anil is so fast a runner that he could not come last in the race.

Dear Student,

The question describes that Anil to be a fast runner and that he would come first in the race. From the answers you have provided the second one has the same meaning as the question and the first one is just opposite of that. Hence, answer would be second one. 

Regards