**Explanation:**

To determine the word that comes first in alphabetical order, we need to compare the first letters of each word.

In this case, the given words are:

a) Exhilarate
b) Ephemeral
c) Entrench
d) Enterprise
e) Enthusiasm

To compare the first letters, we look at the alphabetical order.

- The first letter of "Exhilarate" is "E".
- The first letter of "Ephemeral" is "E".
- The first letter of "Entrench" is "E".
- The first letter of "Enterprise" is "E".
- The first letter of "Enthusiasm" is "E".

As all the words start with the letter "E", we need to look at the second letter to determine the order.

- The second letter of "Exhilarate" is "x".
- The second letter of "Ephemeral" is "p".
- The second letter of "Entrench" is "n".
- The second letter of "Enterprise" is "n".
- The second letter of "Enthusiasm" is "n".

Since "p" comes before "x", "Ephemeral" comes before "Exhilarate".

Now, we need to compare the second letters of the remaining words.

- The second letter of "Entrench" is "n".
- The second letter of "Enterprise" is "n".
- The second letter of "Enthusiasm" is "n".

Since all the remaining words have the same second letter, we need to compare the third letters.

- The third letter of "Entrench" is "t".
- The third letter of "Enterprise" is "t".
- The third letter of "Enthusiasm" is "t".

Again, all the remaining words have the same third letter.

We continue this process of comparing the letters until we find the word that comes first.

- The fourth letter of "Entrench" is "r".
- The fourth letter of "Enterprise" is "e".
- The fourth letter of "Enthusiasm" is "h".

Since "e" comes before both "h" and "r", "Enterprise" comes before "Entrench" and "Enthusiasm".

Therefore, the word that comes first in alphabetical order is "Enterprise" (option D).