In this example question it says which of the following transition metals would you expect to be larger but are actually same or nearly same size as technetium? Well, if we look on our periodic table we have technidium which is in. 5. So if we look just one row down below it in. 6:00, we'd have rhenium.
Remember we saw that Rhenium and Tecnidium have similar atomic radius. That's because of the lanthanide contraction. When we go into the sixth period of the periodic table, we have the introduction of F orbital electrons. The introduction of more of these electrons into one of our inner shells causes a greater attraction between those electrons and are more positive nucleus.
This is termed our effective nuclear charge. So here option D would be our final answer.