There is a multivariate joint PDF like below

And these random variables can be written two vectors such as $K=[x_1{x}_2{]}^{\text{'}}$ and $M=[x_3{x}_4{]}^{\text{'}}.K$ and $M$ vectors are independent with

PDFs$,$

Which of the followings must be placed the gap into the multivariate joint PDF above $(f_{x_{1,\text{d}\text{o}\text{t}\text{s}\text{d}\text{o}\text{t}\text{s}\text{,}},x_4}(x_{1,\text{d}\text{o}\text{t}\text{s}\text{d}\text{o}\text{t}\text{s}\text{,}}{x}_4))?$

L$$tfen birini se$$in:

a$.a^2{b}^2$

b$.a**b$

c$.\sqrt[\phantom{2}]{ab}$

d$.a+b$