Answer:
Second option:

Explanation:
A linear function should have y as a function of x (or x as a function of y) and the degree of the function (the highest power has to be 1)
Both x and y must be present to make it a linear function
General slope intercept form is y = mx + b where m = slope, b = y-intercept
With these restrictions the second option is the correct one

This line is in slope intercept form y = mx + b where m = slope(in this case 1/2) and y-intercept b(in this cae -5)
Option 1: x = 3 is a straight vertical line parallel to the y-axis. Such a line has a slope of ∞
Option 3 is a function with degree 2 and is a quadratic function
Option 4 : 3x - 6 = 4==> 3x = 6 + 4 = 10 or x = 10/3, again another vertical line with slope = ∞