Depending on the question settings, closed-ended question types will either have round radio buttons or square checkbox buttons. The shape of the button determines the number of answer choices that a respondent can choose.

Why some answer choices have round buttons while others have squares?

Round Radio Buttons:

Radio buttons mean that only one answer can be selected by the respondents from the list of choices. All the other answer choices are automatically deselected when a respondent chooses an answer choice.

Checkboxes: 

Checkboxes mean that the respondents are permitted to choose more than one answer from a list of choices.