Assonance is a poetic term which refers to the repetition of an internal vowel sound within a line or lines of poetry, and it serves as a kind of internal rhyme. Please note that the definition specifically refers to sound rather than just repeating a specific letter. For example, the phrase "teeny, weeny yellow polka-dot bikini" contains three examples of assonance. While it is obvious that the double E's in "teeny" and "weeny" are repeated, we get the same sound in the form of an I in the word "bikini." All that to remind you to listen for assonance rather than just look for it, something which suits poetry well since it is best read aloud anyway.
So, you offer four examples from Emily Dickinson's short poem "May-Flower." They are:
Candid in May
Known by the knoll
The first three all contain obvious matching vowels by sight, but again repeating letters do not necessarily mean repeating sounds, as vowels, in particular, can form many different sounds in the English language.
When we consider "aromatic" and "low," we can only compare the O sounds. The O sounds in both words do match, so this is a minor example of assonance, given the presence of so many other vowel sounds.
It is the letter A which the words "candid" and "May" have in common; however, the A sound in "candid" does not match the A sound in "May." Despite the matching letters, then, this is not an example of assonance.
"Known by the knoll" is a bit of a trick, as the first thing we notice is the repetition of the KN, which of course is consonance rather assonance. When we compare the O sounds in these two words, however, the sound is the same. This line does contain assonance.
The two words in "nature forswears," do not have any vowel sounds in common, so there is no assonance.
Your original question implied that there is only one correct choice--that only one of these four lines from the poem contain assonance. Test each line for yourself by reading them aloud and listening to the vowel sounds. The know-knoll line is clear, as is the O sound in aromatic-low. Perhaps the distraction of the two A's in "aromatic" will eliminate this choice for some people, but there is clearly some assonance in these two words.
Known by the knoll,
Next to the robin
In every human soul.
In the context of the poem, "soul" provides another example of the long-O sound in "known" and "knoll," as does "low" in the first stanza.