can't stop multiple radio button selection

Have you specified that they are in a group, they only allow for single selection if they are part of a group.

For example

<input type="radio" name="group1" value="PersonA">
<input type="radio" name="group1" value="PersonB">
<input type="radio" name="group1" value="PersonC">

OK.
They are on different <td> tags but can't i please allow just a single selection?

<td> tags are for tables not for the selection of radio buttons. to group radio buttons together, the names of all the radio buttons should be the same, like in the example I have shown you above.

It would help if you showed your code. How do you expect somebody to help when they can't see your problem?

<tr><td>
student<input name="student" type="radio" value="student" checked tabindex="1"/>
teacher<input name="teacher" type="radio" value="teacher" tabindex="2"/>
other  <input name="other" type="radio" value="other" tabindex="3"/> 
<br/>
<img src="images/student.png" alt="student" width="50px" height="40px">
<img src="images/teacher.png" alt="teacher" width="50px" height="40px">
<img src="images/other staff.png" alt="other" width="50px" height="40px">
</td></tr>

mikulucky already give you the answer.

wow, just got the answer.
The inputs should have the same 'name' value.
thanks