According to the velocity and time graph: 

Choose which one of the following is the magnitude of average acceleration from 8s to 12s.

From the graph, we can see that:

Velocity at time 8s = initial velocity = v₁ = 8 m/s and initial time t₁ = 8s

Velocity time 12s = final velocity = v₂ = 4 m/s and final time t₂ =12s

Average acceleration,

${\mathrm{a}}_{\mathrm{avg}}=\frac{\mathrm{Final} \mathrm{velocity}\left({\mathrm{V}}_2\right)-\mathrm{Initial} \mathrm{velocity}\left({\mathrm{V}}_1\right)}{\mathrm{Final} \mathrm{time}\left({\mathrm{t}}_2\right)-\mathrm{Initial} \mathrm{time}\left({\mathrm{t}}_1\right)}$

         $=\frac{4-8}{12-8}$

${\mathrm{a}}_{\mathrm{avg}}=-1 \mathrm{m}/{\mathrm{s}}^2$