23) B

Butane would always have 4 carbon atoms.
Mr of 4 carbon atoms = 12 x 4 = 48

80% = 48
1% = 48 ÷ 80 = 0.6
100% = 0.6 x 100 = 60

So the mass of butane = 60

Mass that must be made by deuterium and hydrogen = 60 – 48 = 12
Mass of deuterium = 2 x mass of hydrogen

Let number of hydrogen atoms = H
Let number of deuterium atoms = D

H + 2D = 12

Sum of number of hydrogen and deuterium atoms = 10
So H + D = 10

Using simultaneous equations,
H + 2D = 12
H + D = 10
D = 2

Since H + D = 10
H = 10 – 2 = 8

So the formula must be C₄H₈D₂