Growth hormone activity is
Growth hormone (GH) or somatotropin, also known as human growth hormone (hGH or HGH) in its human form, is a peptide hormone that stimulates growth, cell reproduction, and cell regeneration in humans and other animals
It is secreted by the anterior lobe of the pituitary gland. It stimulates the growth of essentially all tissues of the body, including bone. Its activity is increased by thyroxine, produced by thyroid gland.
So, the correct option is ‘Increased by thyroxine’.