"Tedium" refers to the state of being tedious: a feeling of boredom caused by long, dull, or repetitive activity. The question wants the option that matches this sense of dull repetition.
"Monotony" is the word that shares tedium's sense of dull, repetitive boredom.
Let's summarize:
So the correct answer is "Monotony."
If the price of a commodity increases by 25%, by what percentage should the consumption be reduced to keep the expenditure the same?