Instantaneous acceleration at any point on the graph is the slope of the tangent line to the graph at that point. When this line is horizontal and has slope 0 - this when the instantaneous acceleration of the particle is equal to 0. These times correspond to the extrema of the velocity function, which according to the plot occur around t ≈ 1.75, t ≈ 2.75, t ≈ 3.25, and for all t ≥ 4.
I would say t = 2.7 is the most likely choice.
