The correct answer is C. Transformer.
A diode is a semiconductor device that allows current to flow in one direction only. A triode is a three-terminal vacuum tube that can be used as an amplifier, oscillator, or switch. A transistor is a semiconductor device that can be used to amplify or switch electronic signals and electrical power. A transformer is a passive electrical device that transfers energy between two or more circuits through electromagnetic induction.
Transformers cannot be developed within an integrated circuit because they require a magnetic core, which is not possible to create in a small enough size to be integrated with other components.