This is a simple extension of the
Hidden Pair idea. Referring to the bottom
right 3x3 sub-square, you will find that the three highlighted cells are the only ones which contain any of the
numbers 4, 5 and 9. Therefore, 4, 5 and 9 (in some order) must be the solutions for these three cells, and other
numbers in these cells are superfluous and may be removed.
This is a case of a
Hidden Triple within a sub-square, but remember that they can also be found in rows
and columns.