When the function \( f(x) = 2x \) is replaced by \( f(x) + d \) and \( d \) is less than zero, it means you are shifting the graph downwards.
Since \( d \) is negative, you can think of it as moving down by the absolute value of \( d \). Specifically, if \( d < 0 \), then \( -d > 0 \), which indicates a downward shift by that amount.
The correct statement describing this effect is:
The exponential function undergoes a vertical shift down \( -d \) units.
(Note that the specific terminology around “negative d” refers to moving down by the negative of a negative, which results in a positive move downward.)