When an object undergoes acceleration:
a) Its speed always increases
b) its velocity always increases
c)It always falls down towards the earth
d) a net force always acts on it

Hint: All the above options satisfy the conditions when an object is accelerating. Define the acceleration to the general form. Further, we have to manipulate the other options on the basis that in any given circumstances the condition is not violated.

Complete step-by-step answer:
Let us first dine acceleration of a body.
Acceleration is defined as how fast a body changes its velocity with respect to time. Mathematically we can write it as $\overline{a}=\dfrac{\Delta v}{\Delta t}= \displaystyle \lim_{\Delta t \to 0} \dfrac{dv}{dt}$
Now let us manipulate each and every case one by one,
a) Its speed always increases with time
b) Its velocity always increases with time
If we consider a body in circular motion we say that it experiences a centripetal force given by
$\overline{F}=\dfrac{m{{v}^{2}}}{r}$ where m is the mass of the object, v is the speed of the object and r is the radius of the circular path. The object is said to be under centripetal acceleration. Here acceleration is given by $\dfrac{{{v}^{2}}}{r}$ . But in this case v i.e. speed does not change with time. Speaking about the velocity, the magnitude remains the same but its direction keeps on changing in the circular path. Hence it’s not necessary that speed as well as velocity increases if the body is under acceleration .
b)An object falling towards the earth
when an object falls towards the earth there is no doubt it would accelerate due to gravitational force. But it is always not necessary that an object falling towards the earth only accelerated. Like the case a the particle is moving in a circle and is under acceleration
d) A net force always acts on it
From Newton’s second law we state that force on an object is given by i.e.
${{\overline{F}}_{NET}}=m\overline{a}$ where m is the mass of the object and a is its acceleration
Let us say if you are applying a force on a wall or when we stand we apply force on the ground, but neither the wall nor the ground does not move. It is because the net force is zero. Hence, if there is a net force acting on a body it will be under acceleration.
Hence the correct option to the above answer is d.

Note: The rate at which the velocity of an object changes is the acceleration of the object. It is to be noted that the velocity decreases when there is deceleration. Deceleration is equal to acceleration but they differ by minus sign.