The equation 36 ÷ 12 = 3 provides the best estimate for 35.78 ÷ 12.31. This is because 36 and 12 are the closest whole numbers to 35.78 and 12.31, respectively, and dividing them will give a result that is close to the actual value of 35.78 ÷ 12.31. The other equations do not provide as accurate an estimate because they use numbers that are further from the actual values.