Cannot run dropdown from bootstrap

<?php
	session_start();
	include "Inc_db.php";
	//error_reporting(E_ALL ^ (E_NOTICE | E_WARNING));
	//$totalRow=0;

	$sql= "SELECT student_nim, student_name, student_program FROM tb_student";
	$result=mysql_query($sql);
	$totalRow=mysql_num_rows($result);
?>



<html>
<head><title>Mahasiswa</title>

 <link href="Bootstrap/css/bootstrap.css" rel="stylesheet">
  <link href="Bootstrap/css/bootstrap.min.css" rel="stylesheet">



</head>
	<body>
		<h1><center><b>DATA MAHASISWA</b></center></h1>
		<center><table border=1 width=75%></center>		
		<tr>
			Keyword <br>
		</tr>
		<form action="" method="POST">
			Nama:
			<input type="text" name="frm_nama" />
			NIM:
			<input type="text" name="frm_nim" />
			Prodi:
			<select class="custom-select">
  				<option selected>--ALL--</option>
  				<option name="prodi" value="MI" name="frm_mi">D3 Manajemen Informatika</option>
				<option name="prodi" value="SI" name="frm_si">SI Sistem Informasi</option>
				<option name="prodi" value="TI" name="frm_ti">SI Teknik Informatika</option>
			</select>
			<input type="submit" name="frm_cari" value="Cari" class="btn btn-primary btn-sm" />
		</form>
	</br>

		
		<?php
			if(isset($_POST['frm_cari'])){
				$var_name=$_POST['frm_nama'];
				$var_nim=$_POST['frm_nim'];

				$sql="SELECT student_nim, student_name, student_program FROM tb_student WHERE student_nim LIKE '%".$var_nim."%' AND student_name LIKE '%".$var_name."%'";
				$result=mysql_query($sql);
				$totalRow=mysql_num_rows($result);
			}//-if $var_cari
			else{
				$sql=("SELECT * FROM tb_student") or die(mysql_error());
			}
		?>
		<?php
			echo "Jumlah data:$totalRow<br/>";
		?>

<br>
<tr>
<td>NIM</td><td>Nama</td><td>Prodi</td><td>Aksi</td>
</br>
</tr>
<?php
if($totalRow==0){
?>
	<tr><td colspan="4">Data belum tersedia</td></tr>
<?php
}
	else {
	while ($data=mysql_fetch_array($result)) {
		$nim= $data['student_nim'];
		$name= $data['student_name'];
		$program= $data['student_program'];
	
		?>
		<tr>
			<td><?php echo $nim ?></td>
			<td><?php echo $name ?></td>
			<td><?php echo $program ?></td>
		<td><a href="Student_edit.php">Edit</a>
			<a href="Student_delete.php">Hapus</a></br>
			</tr>
		<?php 
	} //-while
}
//}
?>
		</table>
		<form action="Student_insert.php" method="POST">
		<tr>
			<br>
			<td><input type="submit" value="Tambah Data" class="btn btn-primary btn-sm"> </td>
		
		
	</form>
		<form action="Student.php" method="POST">
				<input type="submit" value="Refresh" class="btn btn-primary btn-sm">
		</br>
		</tr>
	</form>
	</body>
</html>

I cleaned up your code.
You need to use triple backticks to post code to the forum.
See this post for details.