Solution (source code)

= Solution
{c}

Obtain each <marginal density> by integrating the <joint probability density function> over the other variable:
$$
\boxed{f_U(u)=\int_0^1u(1+2v)\,dv=2u,\quad0\le u\le1,}
$$
$$
\boxed{f_V(v)=\int_0^1u(1+2v)\,du=\frac12+v,\quad0\le v\le1.}
$$
Each <marginal density> is zero off its displayed <interval> and has total <integral> one.