Assume that the earth goes round the sun in a circular orbit with a constant speed of 30 km/s.

(A.) Given time period is 6 months.

Distance covered by earth will be half the revolution around sun.

Net displacement of earth is not zero.

This implies, average velocity cannot be zero.

(B.)

(C.) It is given that earth moves with a constant speed of 30 km/s. Hence, average speed is 30 km/s and not zero.

(D.) In revolution around sun, the earth moves in a circular orbit around sun which is at the centre. This circular motion is accelerated motion as direction changes at every instant, hence there will be a radial centripetal force acting on earth. Therefore, the instantaneous acceleration of earth is also radial and points towards the centre, i.e. sun.