"The cook cooks a snake."
Translation:Il cuoco cucina un serpente.
There is no difference between "a" and "one"; in this case they are interchangeable.
The rule is that you only use uno if the masculine noun begins with two consecutive consonants (or a z, or there are a few other less frequent cases). Since a vowel follows the "s" in serpente, you must use "un".
uno specchio (double consonant and masculine noun)
uno studente (double consonant but you can't always count on the last letter to indicate the gender of the noun)
un serpente (single consonant at the start, masculine noun)
una scuola (feminine, so the double consonant does not matter for this rule)
Duo must have visit Minahasa Land in North Sulawesi. Because here people there also cook snake (python)