The correct option is C Actinium
The fourth transition series or 6d series corresponding to the filling of 6d sublevel starts with Actinium (Z=89) followed by elements with atomic number 104 to atomic number 112. These elements lie in the seventh period of the periodic table. This series include elements Rutherfordium, Dubnium, Seaborgium, Bohrium, Hassium, Meitnerium, Darmstadtium, Roentgenium, Copernicium.