Option 1: 371,000 units - This is incorrect. Increases of 1.3% in 1987 and a decrease of 6% in 1988 do not result in such a low total.

Option 2: 531,100 units - This is correct. The town T consumed 500,000 units in 1986 and then it increased by 1.3% in 1987. So, 500,000 + (1.3/100)*500,000 = 506,500 units in 1987. Then, the consumption decreased by 6% in 1988, so 506,500 - (6/100)*506,500 = 531,100 units in 1988.

Option 3: 551,100 units - This is incorrect because it results from applying the percentage changes incorrectly.

Option 4: 571,100 units - This is also incorrect as it is greater than the correct answer, indicating that the consumption had been overestimated.

In this type of problem, it`s important to sequentially apply the percentage changes year by year to the previous year`s total.