Is D greater than 1.55

If D is a decimal, is D ≥ 1.55?

"If D is a decimal" = "if D is a real number". So D is a real number and we want to know if it is ≥ 1.55. Perhaps the best strategy here is to just pick numbers and see if we can prove the statements to be insufficient. Since this question is about number rounding, we should test the most extreme values that fit the statements.

(1) When D is rounded to the nearest tenth, the result is 1.6

If we are rounding up to 1.6, we would need to have a number in the form D = 1.5X, where X is the hundredths digit.
To round up, it must be true that X ≥ 5, otherwise we would round down to 1.5
Therefore, it must be true that D ≥ 1.55 --> SUFFICIENT

(2) When d is rounded to the nearest integer, the result is 2

If we are rounding up to 2, we would need to have a number in the form D = 1.X, where X is the tenths digit.
To round up, it must be true that X ≥ 5, otherwise we would round down to 1.
Therefore, it must be true that D ≥ 1.5
If D = 1.5 the answer is NO. If D = 1.55 the answer is YES. --> INSUFFICIENT. The correct answer is A.