Instead of picturing each option's curve from memory, it helps to reason from what the slope of a position time graph actually means. At any instant, the slope of the x-t curve is the velocity, so the overall shape of the graph is really a picture of how velocity changes with time.
Negative acceleration means the velocity keeps falling as time goes on. If the particle starts out moving with some speed, that speed steadily drops, so the slope of the x-t graph must keep getting flatter as we move further along the time axis.
A straight line has a constant slope, which would mean zero acceleration, so that rules out any linear graph. A curve that gets steeper with time has an increasing slope, meaning increasing velocity, which is positive acceleration, the opposite of what is needed here. The only shape left is a curve that starts out steep and bends over, flattening as time passes, that is, one that is concave downward, matching a particle that keeps losing speed.
