The most abundant elements by mass in the body of a healthy human adult are: Oxygen (61.4%); Carbon (22.9%), Hydrogen (10.0%); and Nitrogen (2.6%). The weight which a 75 kg person would gain if all $$^{1}$$H atoms are replaced by $$^{2}$$H atoms is:

The body of the person is said to have a total mass of $$75\;\text{kg}$$.

We are also told that hydrogen contributes $$10.0\%$$ of the body mass. The definition of percentage by mass is:

$$\text{mass of element} = \left(\dfrac{\text{percentage}}{100}\right)\times \text{total mass}$$

Substituting the given numbers for hydrogen, we get

$$m_{^{1}\!H} = \left(\dfrac{10.0}{100}\right)\times 75\;\text{kg} = 0.10 \times 75\;\text{kg} = 7.5\;\text{kg}.$$

Each ordinary hydrogen atom, $$^{1}\!H$$, has an atomic mass of approximately $$1\;\text{u}$$, whereas deuterium, $$^{2}\!H$$, has an atomic mass of approximately $$2\;\text{u}$$. Replacing every $$^{1}\!H$$ with $$^{2}\!H$$ therefore doubles the mass of every hydrogen atom. Mathematically, the new mass of hydrogen becomes

$$m_{^{2}\!H} = 2 \times m_{^{1}\!H} = 2 \times 7.5\;\text{kg} = 15\;\text{kg}.$$

The increase in the person’s total weight is the difference between the new hydrogen mass and the original hydrogen mass:

$$\Delta m = m_{^{2}\!H} - m_{^{1}\!H} = 15\;\text{kg} - 7.5\;\text{kg} = 7.5\;\text{kg}.$$

So, the weight gained after the complete replacement of $$^{1}\!H$$ by $$^{2}\!H$$ is $$7.5\;\text{kg}$$.

Hence, the correct answer is Option B.