<p>A dramatic form congenial to Robert Browning was the dramatic (monologue, it let him explore) a character's mind without the simplifications demanded by stage productions. </p>
<p>The answer to this was:
monologue, which let him explore</p>
<p>I got the answer to be "monologue that lets him explore"
since "let/lets" is referring to the monologue which is singular shouldn't it be "lets"?</p>