1. SelectBox

                 <select class="form-control" id="select_id" name="select_id">
                      <option value="1" >1</option>
                      <option value="1">2</option>
                     
                    </select>
                 

A selectbox should have name and value

2. with label

                 <div class="form-group">
                        <label for="sel1">Select list:</label>
                        <select class="form-control" id="select_id" name="select_id">
                                <option value="1" >1</option>
                                <option value="1">2</option>
                        
                        </select>
                      </div>