Which process consists of a series of linked reactions that amplify an initial signal within a cell?

The correct choice is the process known as a signal cascade. A signal cascade involves a chain of biochemical events initiated by the binding of a signaling molecule, such as a hormone or neurotransmitter, to a receptor on the cell surface. This initial event triggers a series of reactions that lead to a magnified response within the cell. Each step in the cascade often activates the next molecule in the chain, allowing for a small initial signal to produce a significant cellular response, such as changes in gene expression, enzymatic activity, or cellular behavior.

In contrast, cell differentiation refers to the process by which a cell becomes specialized in function and structure, often involving changes in gene expression but not necessarily linked to amplifying a signal. Gene expression is the process by which information from a gene is used to synthesize a functional gene product, typically a protein, but does not inherently describe a series of reactions that amplify signals. A metabolic pathway consists of sequences of chemical reactions in a cell, often involving the conversion of substrates into products, and while these can be linked, they do not emphasize signal amplification in the same way that a signal cascade does.