Calculate the moisture content of a sample given its humid mass (1200g) and dry mass (1050g).

The mass of water is the difference between the humid mass and the dry mass.

$$\text{Mass of water} = \text{Humid mass} - \text{Dry mass}$$ $$\text{Mass of water} = 1200 \text{ g} - 1050 \text{ g}$$ $$\text{Mass of water} = 150 \text{ g}$$

Moisture content ($w$) is the ratio of the mass of water to the dry mass, expressed as a percentage.

$$w = \frac{\text{Mass of water}}{\text{Dry mass}} \times 100\%$$ $$w = \frac{150 \text{ g}}{1050 \text{ g}} \times 100\%$$ $$w = \frac{1}{7} \times 100\%$$ $$w \approx 14.2857\%$$

Rounding to one decimal place, we get $14.3\%$.

Comparing this to the given options:
A) 150g
B) 143%
C) 0.143
D) 14.3%

The correct option is D.

The final answer is \boxed{\text{D) 14.3%}}.