JEE Main · 2020 · 06 Sep 2020 · Shift 1 · d and f Block Elements

Transition elements are those with partly filled $d$ orbitals in the atom or in a common ion. The set $21,25,42,72$ is scandium, manganese, molybdenum and hafnium, all genuine $d$-block transition metals. The other sets contain $s$-block, $p$-block or $f$-block atomic numbers, so only $21,25,42,72$ consists entirely of transition elements.