unable to select one radio button at a time

The name attribute must be the same to select only one radio button at a time. The id could be different depending on if you want to capture that somewhere which one of the radio buttons is selected.

<input type="radio" name="A" value="a" id="radio1"> A &nbsp Option A <br>
<input type="radio" name="A" value="b" id="radio2"> B &nbsp Option B <br>
<input type="radio" name="A" value="c" id="radio3"> C &nbsp Option C <br>
<input type="radio" name="A" value="d" id="radio4"> D &nbsp Option D <br>

To select only one radio button, you have to put them under one group, i.e make the name attribute same for all of them.

<input type="radio" name="options" value="a" id="radio1"> A &nbsp Option A <br>
<input type="radio" name="options" value="b" id="radio2"> B &nbsp Option B <br>
<input type="radio" name="options" value="c" id="radio3"> C &nbsp Option C <br>
<input type="radio" name="options" value="d" id="radio4"> D &nbsp Option D <br>