To find the slope of a line given two points, we can use the slope formula:
\[ m = \frac{y_2 - y_1}{x_2 - x_1} \]
In this case, we have the points (3, 0) and (0, -4). Let's assign:
- \((x_1, y_1) = (3, 0)\)
- \((x_2, y_2) = (0, -4)\)
Now, substitute the values into the slope formula:
\[ m = \frac{-4 - 0}{0 - 3} \]
This simplifies to:
\[ m = \frac{-4}{-3} = \frac{4}{3} \]
So the slope of the line is:
\[ \boxed{\frac{4}{3}} \]
Thus, the correct answer is B. \( \frac{4}{3} \).