Masking is a way for students to visually reduce the population of correct answers as they work through solving a question.
It has no impact on the validation of the question.
Students access the response masking button from the right hand menu in an activity as shown in Figure 1.
Figure 1: Example of response masking with cloze drag and drop.
You can check out a demonstration of this here. To use the response masking you can do so by clicking on Answer Eliminator in the right hand menu.
Response Masking is enabled through the activity editor, it will be disabled by default. To enable go to the Navigation/Control Settings and select enable next to Response Masking.
Figure 2: Response masking in the activity editor
Question types that support response masking
Number Line Drag & Drop
Cloze Drag & Drop
Image with Drag & Drop