If you add certain questions in a Google Form e.g. regarding days of the week, months of the year, sizes, etc. Google will suggest the answers that you may wish to add to the form question. You can choose specific answers from the choices you are given, or choose to add all answers to the form. 

If you type a question that has specific answers, the answers will be suggested under the question. You can click on Add All to add all answers, or click on each answer you wish to add.

For example, if you type a question about days of the week, all weekdays will be suggested:

If you type a question about sizes, all standard sizes will be suggested:

You can remove an answer that you have included in error by clicking on the X to the right of the answer on the form: