Which of the following sets of three forces acting on a body cannot have a resultant of zero?

Block $A$ of mass $4 \; kg$ is to be kept at rest against a smooth vertical wall by applying a force $F$ as shown in the figure. The force required is .......... $N$ $\left(g = 10 \; m/s^2\right)$. (in $\sqrt{2}$)

In the following figure,the object of mass $m$ is held at rest by a horizontal force $F$ as shown. The force exerted by the string on the block is

Mr. $A, B$ and $C$ are trying to put a heavy piston into a cylinder at a mechanical workshop in a railway yard. If they apply forces $F_1, F_2$ and $F_3$ respectively on ropes,then for which set of forces at that instant will they be able to perform the said job?

The adjoining figure shows a force of $40\, N$ acting at $30^o$ to the horizontal on a body of mass $5\, kg$ resting on a smooth horizontal surface. Assuming that the acceleration due to gravity is $10\, ms^{-2}$,which of the following statements is (are) correct?
$[1]$ The horizontal component of the applied force is $20\sqrt{3}\, N$.
$[2]$ The weight of the $5\, kg$ mass acts vertically downwards.
$[3]$ The net vertical force acting on the body is $30\, N$ (upwards).

Define mechanical advantage in a lever.