Which configuration of transformer does NOT affect frequency?

An isolation transformer is specifically designed to transfer electrical power from one circuit to another while electrically isolating the two circuits. Its primary purpose is to separate the output from the input, providing safety and reducing the risk of electrical shock and equipment damage.

The frequency of the electrical signal passing through the transformer is determined by the source feeding the transformer. Both step-up and step-down transformers can change voltage levels but do not alter the frequency of the electrical supply. Since the isolation transformer neither alters the voltage nor the frequency, it is unaffected by the frequency of the AC supply. Therefore, it is the configuration that does not influence frequency, making it the correct answer in this context.

In contrast, step-up and step-down transformers change voltage levels while preserving frequency, but the primary question here focuses on configurations that do not influence frequency at all—that's why the isolation transformer is the right choice.