26) C

We are not sure which isotopes of chlorine and bromine are present in the molecule.

Let Ar of Br in the molecule = M
Let Ar of Cl in the molecule = N
Mr of CH₂BrCl = 128

128 = (12 x 1) + (1 x 2) + (M x 1) + (N x 1)
128 = 12 + 2 + M + N
114 = M + N

The only possible combination of isotopes that can be present in the molecule is: Cl = 35 and Br = 79

Chlorine isotope 35 is three times more common as isotope 37.
Ratio = 3 : 1
Total parts = 3 + 1 = 4
So probability of having isotope 35 in the molecule = ³⁄₄

Bromine isotope 79 is equally as common as isotope 81.
Ratio = 1 : 1
Total parts = 1 + 1 = 2
So probability of having isotope 79 in the molecule = ¹⁄₂

Probability of having Chlorine isotope 35 and Bromine isotope 79 = ³⁄₄ x ¹⁄₂ = ³⁄₈