您<td>在选择之前关闭。<select> 应该嵌套在你的<td>.

例如

<td>
    <select name="portoptions[]" multiple="multiple">
        <option value="TCP">TCP</option>
        <option value="UDP">UDP</option>
        <option value="protocol">Protocol</option>
    </select>
</td>

two issues are here. 1. You should put </td> after </select>. 2. You should put one <tr> before <td>.

So your final code will be

<table>
<tr>
    <td>Ports to be opened(<em><small>Shift Select Multiple Options</em></small>:</td>
    </tr>
<tr>
    <td>
       <select name="portoptions[]" multiple="multiple">
           <option value="TCP">TCP</option>
           <option value="UDP">UDP</option>
           <option value="protocol">Protocol</option>
       </select>
    </td>
</tr>
</table>

利用，

<table>

<td>Ports to be opened(<em><small>Shift Select Multiple Options</em></small>:</td>
</tr>

<tr>
<td><select name="portoptions[]" multiple="multiple">
<option value="TCP">TCP</option>
<option value="UDP">UDP</option>
<option value="protocol">Protocol</option>

</select></td>
</tr>

代替：

<td><select name="portoptions[]" multiple="multiple"></td>
<option value="TCP">TCP</option>
<option value="UDP">UDP</option>
<option value="protocol">Protocol</option>

</select>  

至：

<td>
<select name="portoptions[]" multiple="multiple">
<option value="TCP">TCP</option>
<option value="UDP">UDP</option>
<option value="protocol">Protocol</option>
</select>
</td>

所有的控制必须在关闭前结束</td>

通过以您的方式编写，浏览器不知道这些选项。

所以你需要写在两者之间<td> <select><option>...</option></select></td>

通过以上述方式编写您的代码，

<td>
    <select name="portoptions[]" multiple="multiple">
        <option value="TCP">TCP</option>
        <option value="UDP">UDP</option>
        <option value="protocol">Protocol</option>
    </select>
</td>