Which of these hormones is countered by diuretics such as alcohol?

The answer would be: antidiuretic hormone (ADH)

Alcohol will inhibit the secretion of antidiuretic hormone by the pituitary. Diuretic means an agent that causes urine production

As its name says, the antidiuretic hormone will reduce the production of urine. Since this hormone decreased by alcohol, the production of urine will be increased.