<select id = "ejemplo">
<option>Seleccione un valor</option>
<option value = "a">A</option>
<option value = "b">B</option>
<option value = "c">C</option>
</select>
A <input type = "checkbox" name = "a" class = "check" />
<div class = "a">
A1 <input type = "checkbox" />
A2 <input type = "checkbox" />
A3 <input type = "checkbox" />
</div>
B <input type = "checkbox" name = "b" class = "check" />
<div class = "b">
B1 <input type = "checkbox" />
B2 <input type = "checkbox" />
B3 <input type = "checkbox" />
</div>
C <input type = "checkbox" name = "c" class = "check" />
<div class = "c">
C1 <input type = "checkbox" />
C2 <input type = "checkbox" />
C3 <input type = "checkbox" />
</div>