Arranging the words in alphabetical order:
To determine which word comes first in alphabetical order, we need to compare the letters of each word from left to right until we find a difference. Let's arrange the given words in alphabetical order:

1) Centre
2) Constant
3) Consent
4) Cotton

Comparing the first letters:
- Centre: The first letter is 'C'.
- Constant: The first letter is also 'C'.
- Consent: The first letter is 'C' as well.
- Cotton: Again, the first letter is 'C'.

As the first letter is the same for all the words, we move on to the second letter.

Comparing the second letters:
- Centre: The second letter is 'e'.
- Constant: The second letter is 'o'.
- Consent: The second letter is 'o'.
- Cotton: The second letter is 'o'.

Here, we find a difference in the second letter. Since 'e' comes before 'o' in the alphabetical order, 'Centre' comes before 'Constant', 'Consent', and 'Cotton'.

Therefore, the word that comes first in alphabetical order among the given options is 'Centre' (option A).

Summary:
- Centre, Constant, Consent, and Cotton are the given words.
- When arranging them in alphabetical order, we compare the letters from left to right.
- As the first letter is the same for all the words, we move on to the second letter.
- The second letter of 'Centre' is 'e', while 'Constant', 'Consent', and 'Cotton' all have 'o' as their second letter.
- Since 'e' comes before 'o' in the alphabetical order, 'Centre' comes first.
- Therefore, the correct answer is option A.