To develop easy-to-use products, it is important to understand the users and their tasks.

Option A is the correct answer. Understanding the users and their tasks helps to design products that are intuitive, easy to use, and meet the users' needs. This involves conducting user research, usability testing, and user feedback to ensure the product is user-centered and functional.

Option B, following standard guidelines, is also important, but it is not the most important factor. Standard guidelines provide a framework for designing interfaces, but they do not always consider the unique needs and preferences of the users.

Option C, making interfaces as consistent as possible, is also important, but it is not the most important factor. Consistency helps to make the product predictable and familiar to the users, but it is not enough to make the product easy to use.

Option D, Internal Quality Checking, is not related to making products easy to use. Internal quality checking is important for ensuring the quality of the product, but it does not guarantee that the product is easy to use.

Therefore, the correct answer is: A. Understanding the users and their tasks.