Using short-cut:
I. Profit = 10%.
II. Using 900g but charging for 1kg $\Rightarrow$ $\frac{1000 - 900}{900} \times 100 \approx 11.11%$.
III. 10% impurities at cost price means selling only 90% sugar for 100% price $\Rightarrow$ $\frac{100 - 90}{90} \times 100 \approx 11.11%$.
IV. Price increase 5%, weight decrease 5%: Net $%$ gain $\approx 5 + \frac{5}{95} \times 100 \approx 10.26%$.
Thus II and III give highest (~11.11%), but all three II, III, IV>10%.
% Quick tip