The horizontal asymptote of an exponential decay function typically indicates the value that the function approaches as the input (usually represented as \( x \)) approaches infinity.
In many exponential decay functions of the form \( y = ae^{-bx} + c \), where \( a \) is a coefficient, \( b \) is a decay constant, and \( c \) is a constant added to the function (which shifts the graph vertically), the horizontal asymptote is determined by the value of \( c \).
Without the actual graph to visualize, the most common horizontal asymptote for exponential decay functions, particularly when they do not shift vertically, is \( y = 0 \).
Hence, if the function approaches zero as \( x \) goes to infinity, we conclude that the horizontal asymptote is at \( y = 0 \).
Therefore, among the provided responses, the most appropriate answer is:
The horizontal asymptote is at \( y = 0 \).