Part A: the answer is C. They only converted milligrams to grams instead of going to kilograms. There are 6 steps between milligrams and kilograms, so 25,000 should've been divided by 1,000,000.
Part B
9.58 kg+1.25 kg+0.025 kg = 10.855 kg
So the bag, water bottle, and sleeping bag weigh 0.145 kg less than 11 kg