By default, the iPhone automatically converts straight quotes like this:  ‘
into smart quotes, like this ` .  You need to turn this functionality off in order to enter the prime (straight quote) symbol in WeBWorK.  The short version is:

To turn smart punctuation off, all you have to do is go to Settings > General > Keyboard, and then turn off the “Smart Punctuation” toggle.

More detailed instructions can be found here: