Track the water weight directly instead of the dry matter.
At the start, water in the grain is 25% of 1000 kg, that is 250 kg, leaving 750 kg of dry solids.
At the end, moisture is 20% of the new total weight, call it \( W_2 \). The dry solids are still 750 kg, and they now form 80% of \( W_2 \), so \( W_2 = 937.5 \) kg, and the water left in the grain is \( 0.20 \times 937.5 = 187.5 \) kg.
Water removed is the drop in water weight, \( 250 - 187.5 = 62.5 \) kg, matching the first method exactly.
Rounded to the nearest option on the list, this lands on 75 kg.
\[ \text{Water evaporated} \approx \boxed{75 \text{ kg}} \]