Answer:
B
Explanation:
A linear function is described by a x term to the power of 1 or even 0. A function when x is to the second power is most usually not a linear function.
In answer A, you have x multiplied against -3x. This will cause an x squared term, so this is not a linear function.
In answer B, you have 1/2 multiplied against x, which is a regular x term subtracted by 2x, causing a regular x term, meaning this function is linear. However, in the small possibility it is not, we will keep going.
In answer C, there is an x squared term so this is not linear.
In answer D, the expression has 1/2x times 2x, which will cause an x squared term.
Thus, the answer is B, which is the only equation without an x squared term.