You're on the right track! Transition elements, also known as transition metals, begin to fill the d sublevel. The d sublevel is the third energy level in an atom and can hold up to a maximum of 10 electrons. As you move across the periodic table from left to right, the d sublevel starts to fill in the fourth period (row). This is where you'll find the transition elements. So, to answer your question, the sublevel that begins to fill with transition elements is the d sublevel.
