If this is a linear function, f(x) = (-1/2)x+15, then the domain is the set of all real numbers. Notice how x is not in the denominator of the fraction.
If x is in the denominator of the fraction, then you would have the domain be the same thing but x = 0 is not allowed.
If (2x+15) is in the denominator, then x = -7.5 is not allowed
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Based on your answer choices, only B seems to fit. So I'd go with choice B as the final answer