Recall that velocity is the change in displacement, divided by the time taken for that change: \(v = \frac{dr}{dt}\). So, graphically, velocity is nothing but the slope of the position-time graph.
Look at the slopes at the 4 different points on the graph for the falling object. Just by looking at the figure, choose the time at which the tangent has the highest slope (ignore the negative sign for now).

\(t = 1 \, \mathrm{s}\)

\(t = 2 \, \mathrm{s}\)

\(t = 3 \, \mathrm{s}\)

\(t = 4 \, \mathrm{s}\)