Which of the following describes iambic pentameter?

Iambic pentameter is indeed characterized by a line of poetry that consists of 10 syllables. This metric form is made up of five pairs of syllables, known as iambs, where each iamb consists of one unstressed syllable followed by one stressed syllable. This rhythmic structure creates a natural flow in the poetry, often mimicking the rhythm of spoken English and contributing to its musical quality.

The other options describe different poetic concepts. Unrhymed verse of irregular length refers to free verse, which does not adhere to a specific syllabic count or rhyme scheme. A stanza of three quatrains pertains to a specific arrangement of verses but does not define a rhythmic meter like iambic pentameter. Lastly, two lines of rhymed verse describe a couplet, which is a distinct poetic form, again unrelated to the specific rhythmic structure of iambic pentameter.