Use `linkText` element Locator method instead of XPath.

        driver.get("http://awqaf.texpo.com/"); 
        WebDriverWait wait = new WebDriverWait(driver, 10);
        WebElement element =wait.until(ExpectedConditions.presenceOfElementLocated(By.linkText("ABOUT US"))); 
       // WebElement aboutdrop=driver.findElement(By.linkText("ABOUT US"));

Also, try with removing `driver.switchTo().defaultContent();` this line.

Generally `switchTo().defaultContent();`  method is used to go back on previous working window. So if it requires in your case then & then you may use.