Which is the lightest transition element?

Sc

Scandium (Sc) is the lightest transition element with an atomic number of 21 and an atomic mass of 44.955912 u.

The other options are incorrect. Fe (iron) has an atomic number of 26 and an atomic mass of 55.845 u. Os (osmium) has an atomic number of 76 and an atomic mass of 190.23 u. Pd (palladium) has an atomic number of 46 and an atomic mass of 106.42 u.

So the answer is (2), Sc.