Explanation:

The microprocessor was introduced in the Fourth Generation of computers. The first commercial microprocessor, the Intel 4004, was released in 1971. This invention marked a revolutionary shift, allowing an entire central processing unit (CPU) to be placed on a single integrated circuit chip. This led to the development of personal computers and distributed computing. The (a) First generation used vacuum tubes. The (b) Second generation used transistors. The (c) Third generation used integrated circuits (ICs) but not microprocessors as the sole CPU. Therefore, the Fourth generation ($ \text{d} $) is defined by the microprocessor.