Final Answer:
Cinquain is the form of poetry that always consists of five lines.
Explanation:
The cinquain, as the name suggests (derived from the French word for "five"), is a poetic form that inherently comprises five lines. This structured form of poetry typically follows a syllable pattern: the first line has two syllables, the second has four, the third has six, the fourth has eight, and the fifth line returns to two syllables. This fixed structure of syllable count across the five lines distinguishes the cinquain form from other poetic structures.
On the other hand, a limerick is a five-line poem, but not all limericks strictly adhere to a fixed syllable count per line. They follow a specific rhyming pattern (AABBA) and often have a distinct rhythm but do not have a fixed syllable count in each line. Haikus, while also consisting of five lines, follow a syllable pattern of 5-7-5 syllables, respectively, in their three lines, making them distinct from cinquains.
Therefore, among the options presented, only the cinquain form consistently maintains a structure of five lines with a predetermined syllable count in each line, distinguishing it as the poetic form exclusively composed of five lines.