Which discovery did scientists make when they arranged elements in order of increasing atomic mass?

a.
a repeating pattern of properties

b.
a puzzling series of numbers

c.
alphabetical arrangement of the names of elements

d.
a complex code of patterns and relationships

The answer to this question would be: A. a repeating pattern of properties 

If you examine the periodic table closely, you will found that the first and second columns of the table are composed of metal. The most element on the right side would be gas too. The table shows that there is a pattern that can predict an element has properties