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 correct option is A.
When scientists arranged elements in order of increasing atomic mass, they discovered that some properties of these elements are repeated in a regular manner and this discovery was used to form the periodic table. For instance, you will notice that all the elements arranged in each group have similar chemical properties