If each orbital can hold a maximum of three electrons, the number of elements in 9th period of periodic table (long form) are :

Number of total orbital,
$1+3+5+7+9=25$

As each orbital can hold a maximum of three electrons, 

No. of elements,

 $$\begin{gathered} =25\times 3 \\ =75 \end{gathered}$$

Hence, the option D is correct.