The correct option is D β−,β−,α,β−
We know that mass number and atomic number of the parent nucleus decrease by 4 units and 2 units in each alpha decay. Whereas mass number remains the same and atomic number of parent nucleus increases by 1 unit in each beta minus decay.
Thus order of decay in the above given reaction is
β−, β−, α, β−
So, correct answer is option D.