The hormone that increases fertility in males is testosterone.
Explanation: Testosterone is the primary male sex hormone, crucial for sperm production and development of male reproductive organs.
Why other options are incorrect:
Oestrogen:
Oestrogen is the primary female sex hormone, not involved in male fertility.
Insulin:
Insulin is a hormone that regulates blood sugar levels, not related to male fertility.
Growth hormone:
While important for overall development, growth hormone is not directly involved in male fertility.