$(i) $ $1+x$

$\because$ The degree of $1+x$ is $1$                                 $\therefore$ It is a linear polynomial.

$(ii)$ $3 t$

$\because$ The degree of $3 t$ is $1 .$                                $\therefore$ It is a linear polynomial.

$(iii)$ $r^{2}$

$\because$ The degree of $r^{2}$ is $2$                              $\therefore$ It is a quadratic polynomial.

$(iv)$ $7 x^{3}$

$\because$ The degree of $7 x ^{3}$ is $3 .$                    $\therefore$ It is a cubic polynomial.