This is the 561st most frequent Ukrainian word.
однією
The Ukrainian word "однією" means "one" (instrumental singular feminine form).
In this sentence, "однією" ("one") is used as an instrumental case of the numeral, describing possession or accompaniment with one flower.
Вона зайшла до кімнати з однією квіткою.
She entered the room with one flower.
In this context, "однією" specifies one element from a group (reasons) and is in instrumental case used in conjunction with the preposition.
Однією з причин цього є нестача часу.
One of the reasons for this is the lack of time.
Here, "однією" refers to a singularly identified item that is instrumental in solving the problem, using a similar grammatical case as in the first sentence.