Today's small, portable computers differ greatly from the earliest electronic computers, which were typically what?

The correct choice highlights a significant characteristic of the earliest electronic computers: their large size. These computers were often housed in rooms and required substantial amounts of space, resources, and cooling equipment due to their complex circuitry and component density. In contrast, today's small, portable computers have evolved to be compact and efficient, allowing for greater mobility and convenience.

The other options, while they could apply to a variety of devices, do not specifically capture the defining physical trait of earlier computers. For instance, 'effective,' 'invented,' and 'useful' do not accurately convey the straightforward fact about their size, which is crucial for understanding how technology has advanced from bulky hardware to the sleek, portable designs we see today. This evolution reflects the ongoing innovations in design, materials, and technology.