Explanation:

The evolution of computing devices progressed through several stages before the advent of what is formally defined as '1st generation computers' (characterized by vacuum tubes, e.g., ENIAC). Initially, there were (a) Mechanical computers like Babbage's Difference and Analytical Engines. These were followed by (b) Electro-mechanical computers, such as the Harvard Mark I and Zuse Z3, which used a combination of electrical relays and mechanical components. Immediately preceding the fully electronic 1st generation, there were early (c) Electrical (electronic) computers like the Atanasoff-Berry Computer (ABC) and Colossus. These machines utilized electronic components (vacuum tubes) but often lacked the general-purpose programmability or full digital architecture that defined the 1st generation. Thus, 'Electrical' ($ \text{c} $) refers to these pioneering electronic machines that paved the way for the first generation.