Concept:
The equation of any straight line can be found using the point-slope form: $y - y_1 = m(x - x_1)$, where $(x_1, y_1)$ is a point on the line and $m$ is the slope. A slope of zero indicates a perfectly horizontal line, meaning the $y$-value remains constant regardless of the $x$-value.
Step 1: Identify the given parameters.
The line passes through the point $(x_1, y_1) = (-3, 7)$.
The slope of the line is given as $m = 0$.
Step 2: State the point-slope formula.
The standard point-slope equation is:
$$y - y_1 = m(x - x_1)$$
Step 3: Substitute the parameters into the formula.
Plug the known values into the equation:
$$y - 7 = 0(x - (-3))$$
Step 4: Simplify the right side of the equation.
Because the slope is zero, the entire right side of the equation is multiplied by zero, neutralizing the $x$ term:
$$y - 7 = 0$$
Step 5: Isolate y to find the final equation.
Add 7 to both sides of the equation to put it in its simplest form:
$$y = 7$$
Hence the correct answer is (B) $y=7$.