Which of the following is a first order reaction /

Question

$\begin{array}{1 1} 2HI \to H_2 +I_2 \\ 2NO_2 \to 2NO +O_2 \\ 2NO+O_2 \to 2NO_2 \\ NH_4NO_2 \to N_2+2H_2O \end{array}$