Questions can be made optional or mandatory. If they are marked as required, respondents will have to provide an answer before they are allowed to proceed to the next page, or submit their survey.
In grid type questions (multiple choice grid, checkbox grid, text response grid, n-atrix, 3d matrix), each row can be made to be optional or mandatory on an individual basis. For example, you could have two rows within a grid be required, while another two are optional.
Validation can be set up on text response fields to assure that the responses adhere to a certain set of parameters. For example, if you want to assure that respondents enter a phone number, you can do so.
Validation can be implemented for:
Check out this tutorial video for more information on Validation:
Check out this documentation for more information.