Final answer:
Hormones mediate changes by activating specific gene expressions when they bind to intracellular hormone receptors. They can also trigger signaling pathways by binding to surface receptors.
Step-by-step explanation:
Hormones mediate changes in the body through several mechanisms. Hormones cause cellular changes by binding to receptors on or inside target cells. The mechanism of action depends on the hormone's chemical nature and the location of the receptor. Lipid-soluble hormones, also known as steroid hormones, can diffuse across the plasma membrane and bind to receptors inside the cell, which can modulate gene expression by directly affecting DNA transcription. This action can change the cell's long-term structure and function through the production of proteins. In contrast, lipid-insoluble hormones, such as peptide and glycoprotein hormones, bind to surface receptors and initiate a signaling pathway. These binding events activate G-proteins, which in turn generate a second messenger such as cyclic AMP (cAMP), leading to a short-term cellular response such as the production of proteins and enzymes or alterations in membrane permeability.
Regarding the student's question, hormones mediate changes primarily by activating specific gene expressions (option b), as hormone receptors can act as transcription regulators when hormones are present and bind to intracellular receptors. Alterations in DNA itself are not a direct function of hormones, therefore option a is incorrect. While structural alterations (option c) and enhancements to the immune system (option d) can be secondary effects of hormonal activity, they are not the primary ways that hormones mediate changes.