What is common to all the characters listed in the said table is that they can be easily seen with the physical eyes. In other words, they are easily observable.
The characters listed in the first column of the table which includes height, flower color, flower position, pod color, pod shape, seed color, and seed shape are those that do not require special instruments to be seen or observed.
They can be observed with unaided eyes even by a layman. The easily observable traits make it easy for such a plant to be utilized in plant breeding experiments.
More on Mendel's plant can be found here: https://brainly.com/question/25294791?referrer=searchResults