Explanation:

- The price of sugar increases by 15%, meaning that for every 100 units of currency spent, one now spends 115 units.

- The goal is to keep the total expenditure constant despite this increase in price.

- Thus, we need to reduce the quantity purchased, so that the expenditure remains the same.

- The relationship between price and quantity when the total expenditure is to remain constant is inversely proportional.

- To find the percentage decrease in consumption, we use the formula for percentage decrease: $(\frac{\text{New Price}-\text{Old Price}}{\text{New Price}})\times 100\mathrm{\%}.$

Given that:

- Old Price = 100 (arbitrary units)

- New Price = 115 (after 15% increase)

- The necessary decrease in quantity is given by: $\frac{15}{115}\times 100\mathrm{\%}=13.04\mathrm{\%}$.

- Therefore, the correct option for the percentage decrease in consumption is:

Option 4: 13.04%.