What is a common characteristic of the transitions elements? the next to outermost electron shell begins to fill up. the next to outermost shell is always complete. they can all be metals or non-metals. all of the above

The next to outermost electron shell begins to fill up.

Explanation:

Transition elements are a group of elements that are present in the center of the periodic table. These are essentially metals that occupy groups 3-12.

They are also known as the d-block elements as it is the inner d-orbital that gets progressively filled with electrons on moving from left to right across a period while the outer s-orbital remains filled.  The general electron configuration is: (n-1)d¹⁻¹⁰ns²

Thus the common characteristic of the transitions elements is that the next to outermost electron shell begins to fill up i.e. the d orbitals fill up relative to the outer s-orbital