First, find the molar mass of $Ca(OCl)_2$.
Calcium: $40$.
Oxygen: $2 \times 16 = 32$ (because there are two O atoms).
Chlorine: $2 \times 35.5 = 71$ (because there are two Cl atoms).
Total molar mass = $40 + 32 + 71 = 143$.

The percentage by weight of calcium is

$$\frac{40}{143} \times 100\%.$$

Calculate:
$40 \div 143 \approx 0.2797$,
$0.2797 \times 100 \approx 27.97\%$,
which rounds to $28.0\%$.

Option B (31.6%) might come from misplacing the decimal or using an incorrect molar mass.

So the correct answer is A) 28.0%.

Textbook reference: Comprehensive Certificate Chemistry, p. 66