Which of these is essential in a poem?
rhyme
rhythm
verses
nothing is essential in a poem except that the language is condensed and powerful

Rhyme is considered as a essential thing in a poem.

Option A

Explanation:

Poetry in olden days had certain rules to be followed. A poem is a rhyme with regular rhythm. In a poem it is essential to have Rhymes, rhythm, meaningful verses and a powerful language out of which rhyme is very important. A poem generally exhibits a standard rhyme scheme.

Actually rhyming words are used in a paragraph in alternative lines. This gives a kind of zest for the readers to read out. This is found at the end of lines and generally exhibits a rhythm when read out.  

"a.) well–developed vascular system."

a unit of heredity that is transferred from a parent to offspring and is held to determine some characteristic of the offspring.