Final answer:
The fixed form described in the question, featuring five lines with an anapestic rhythm and a 'aabba' rhyme scheme, is a limerick.
Step-by-step explanation:
The fixed form of five lines with an anapestic rhythm and a rhyme scheme of aabba is known as a limerick. A limerick is a humorous poem consisting of a single stanza of five anapestic lines with the third and fourth lines shorter than the others. It adheres to a strict rhyme scheme and is known for its amusing content and bouncy rhythm, which is characteristic of its anapestic meter, where two short syllables are followed by a stress syllable. This description does not match the structure of the other options listed, such as a sonnet, haiku, or ballad, which all have distinct forms and rhythm patterns different from the limerick.