Answer: dataset Z
The genus of aquatic plants known as Elodea is composed of 6 species that are also called waterweeds. Native to South and North America, these plants are very popular in aquariums.
The table in the image presents the stem growth of plant stems in 22 days. In three of the trials, the stems grow constantly. However, in dataset Z the growth reaches a maximum in day 15, followed by a decrease in day 22.
As nothing additional is mentioned in the question about the presence of any possible species that may feed on these plants, this decrease is anomalous. As it is absent in the other trials, it is safe to assume that a measurement error has occurred. Thus, dataset Z has questionable data.
