Straight line depreciation applies the same amount of depreciation in each year.
Our Depreciation Base is 21,000 - 1,000 = 20,000
The useful life is 5 years, so each year we depreciate 20,000 รท 5 = 4,000
Book Value is Cost - Accumulated Depreciation
After Year 1:
Book Value = 21,000 - 4,000 = 17,000
Answer is A) 17,000