AIEEE 2003 Chemistry Solved Paper

This cant be f-block elements as f block means only lanthanides and actinides but we know lanthanide contraction happens on the elements before lantanidine elements also. So, radius also decrease for those element when increase the atomic number.
Radioactive Series starts from atomic number 84 (Polonium) and affer that all are radioactive. But we know before 84 atomic number also the atomic size decrease with increase in atomic number. So, only radioactive series don't show this characteristic.
So, correct answer should be high atomic mass.