Modular PLCs can be expanded by adding:

Modular PLCs (Programmable Logic Controllers) are designed to be flexible and expandable to accommodate a variety of automation needs. The correct answer, which is the ability to expand by adding cards, reflects this design principle.

In a modular PLC system, expansion is typically achieved through the addition of various input/output cards, or modules, that fit into the main PLC unit. These cards can be specifically tailored for different functions, such as analog input/output, digital input/output, relay output, or communication interfaces. This flexibility allows a PLC to be configured exactly according to the requirements of the application, enabling the addition of extra functionality without the need for a completely new controller.

While memory upgrades can enhance the capabilities of a PLC, they do not necessarily change the number of input or output channels directly. Similarly, while additional inputs and outputs can be added, they often require corresponding expansion cards. Hence, the most direct method for enhancing the functionality of a modular PLC system is indeed the addition of cards.