Select selectObject = new Select (driver.findElement(By.xpath("//select[@name='brand']")));
    selectObject.selectByValue("MC");

    driver.manage().timeouts().implicitlyWait(30, TimeUnit.SECONDS);

    Select selectObject2 = new Select (driver.findElement(By.xpath("//select[@name='prefix']")));
    selectObject2.selectByValue("546");

<SELECT onchange="document.form2.elements['stdate'].value=document.form1.elements['stdate'].value;document.form2.elements['gsmnotype'].value=document.form1.elements['gsmnotype'].value;document.form2.elements['brand'].value=document.form1.elements['brand'].value;document.form2.elements['prefix'].value=document.form1.elements['prefix'].value;document.form2.submit()" name=prefix> <OPTION value=53>53</OPTION><OPTION value=541>541</OPTION><OPTION selected value=542>542</OPTION><OPTION value=543>543</OPTION><OPTION value=50>50</OPTION><OPTION value=544>544</OPTION><OPTION value=545>545</OPTION><OPTION value=546>546</OPTION><OPTION value=549>549</OPTION><OPTION value=55>55</OPTION></SELECT>