The correct answer is C. User-friendly.
A user-friendly interface is one that is easy to use and understand. It should be designed with the user in mind, and should be intuitive and easy to navigate. A user-friendly interface should also be consistent, so that users know what to expect and can easily find what they are looking for.
A user-happy interface is one that makes the user feel happy or satisfied. This could be achieved through the use of positive colors, images, or sounds. However, a user-happy interface is not necessarily user-friendly. For example, a website that is full of bright colors and animations may be user-happy, but it may also be difficult to use and navigate.
A user-simple interface is one that is easy to understand and use. This can be achieved by using simple language, clear instructions, and easy-to-follow steps. However, a user-simple interface is not necessarily user-friendly. For example, a website that is full of simple text and images may be user-simple, but it may also be boring and uninteresting.
Therefore, the correct answer is C. User-friendly.