Which of the following is true for elastic potential energy density?

$A$ metal wire of length $L$ is suspended vertically from a rigid support. When a body of mass $M$ is attached to the lower end of the wire,the elongation in the wire is $l$. Consider the following statements:
$(I)$ The loss of gravitational potential energy of mass $M$ is $Mgl$.
$(II)$ The elastic potential energy stored in the wire is $Mgl$.
$(III)$ The elastic potential energy stored in the wire is $\frac{1}{2} Mgl$.
$(IV)$ The heat produced is $\frac{1}{2} Mgl$.
Which of the following statements are correct?

What is called elastic energy density? Write its formula and dimensional formula.

$A$ wire of length $L$ and area of cross-section $A$ is made of a material with Young's modulus $Y$. It is stretched by an amount $x$. The work done in stretching the wire is:

$A$ wire suspended vertically from one end is stretched by attaching a weight $200 \,N$ to the lower end. The weight stretches the wire by $1 \,mm$. The elastic potential energy gained by the wire is ....... $J$

$A$ thick rope of density $\rho$ and length $L$ is hung from a rigid support. The Young's modulus of the material of the rope is $Y$. The increase in length of the rope due to its own weight is