Final answer:
The formula for long-run investment elasticity in relation to user cost of capital is the percentage change in investment over the percentage change in user cost of capital, not to be confused with the slope of a curve, which is a different concept.
Step-by-step explanation:
The correct formula for the long-run elasticity of investment with respect to the user cost of capital is given by the option C: The percentage change in investment divided by the percentage change in the user cost of capital. This measure of elasticity indicates how much the quantity of investment will respond to a change in the user cost of capital. Remember that elasticity is different from the slope of a curve; while the slope is a simple rise over run measure, elasticity involves the comparison of percentage changes, providing a dimensionless measure of responsiveness that can be validly compared across different contexts.