What is the function to get plugin base url in multisite?

The plugins_url() function is designed to work in WordPress Multisite. Here is an example of its proper usage:

<?php
echo "<img src="' .plugins_url( 'images/wordpress.png' , __FILE__ ). '" > ';
?>

If you are using this function in a file that is nested inside a
subdirectory, you should use PHP’s dirname() function:

<?php
echo "<img src="' .plugins_url( 'images/wordpress.png' , dirname(__FILE__) ). '" > ';
?>