<em>A</em><em> </em><em>la</em> is the right answer that you're looking for because the other options don't make sense at all :)
Also, El clase is singular, so if it'll be <em>a</em><em> </em><em>la</em><em>.</em><em> </em>If it was los clases, it would've been <em>a</em><em> </em><em>las</em><em>.</em><em> </em>
