Answer:
Iambic pentameter.
Explanation:
Iambic Pentameter is the construction of lines in poetry with five iambs/ metrical feet. These meter scheme is also called as pentameter, where an stressed syllable follows an unstressed syllable in all the lines.
"On First Looking Into Chapman's Homer" by John Keats is an Italian sonnet that has a perfect rhyme scheme of abbaabba for the first eight lines. the eighth line is the volta, from where the tone changes and the rhyme scheme also changes to cdcdcd.
This poem uses the iambic pentameter form of meter, whereby the lines contain ten syllables, an unstressed syllable followed by an unstressed syllable. Example-
<em>Much </em><em>have</em><em> I</em><em> tra</em><em>vell'd </em><em>in</em><em> the </em><em>realms</em><em> of</em><em> gold,
</em>
<em>And </em><em>ma</em><em>ny </em><em>good</em><em>ly </em><em>states</em><em> and</em><em> king</em><em>doms </em><em>seen;</em>
The stressed syllables are in bold, alternating in between unstressed syllables. Thus, the base form of the poem "On First Looking Into Chapman's Homer" is iambic pentameter.