how to select one radio button at a time

in the below code, i am trying to display radio-buttons. the problem is, when i run the code there is possibility to check all the radio-buttons at the same time. what i want to achieve is only one radio button is able to be checked at a time.in other words, the
user should not be able to select more than one radio-button, only one is selected and the rest should be unchecked.
please let me how to adapt the below code to achive that