Correct Answer - D
Displacemeant in `12 s ` =Area undr `v-t` groph
`=(1)/(2) xx (12+5) 4=34 m`
`V_(av) =(Desplacement )/(Time ) =(34)/(12) =(17)/(6) m s^(-1)`
Hence, (a) is incorrect, (b) is incorrect because during first `3 s`, velocity increases from= to `4 m s^(-1)` option, (c ) is incorrect, because in part `AB` velocity is constant.