By default, the iPhone automatically converts straight quotes like this:Β  ‘
into smart quotes, like this ` .Β  You need to turn this functionality off in order to enter the prime (straight quote) symbol in WeBWorK.Β  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 β€œSmart Punctuation” toggle.

More detailed instructions can be found here:

https://www.howtogeek.com/344310/how-to-turn-off-smart-punctuation-on-your-iphone-and-ipad/