You read a graph from left to right, just like you would read words on a page in a book. At the left-most "side" of this graph, we are way up at positive infinity. As we move from left to right, the graph begins to drop in y values. A drop in y values indicates a decreasing graph. So it's decreasing, we have that part. As the graph grows up into y values that increase forever, it will also grow in the x direction to negative infinity. As the graph decreases in y values from left to right, the x values increase into positive infinity. So your choice is the second one: The function is decreasing on the interval (-∞, ∞).
