FYI, that L in the denominator is factorial.
This is some pretty serious stuff for high school.
My inclination is that the answer is A, which is the definition of <em>e. </em> Let's see if we can show this.
Let's write
Let's expand this with the binomial expansion
Let's focus on when n is really big and on the ks that are relatively small, which make up the bulk of the sum as the terms get small rapidly.
Then that numerator n(n-1)···(n-k+1) ≈ n^k as all the factors are about n.
OK, we showed for large n this is approximately true, and it will be exactly true in the limit, so we choose
Answer: A