Which equation is correct for first order reactions?

The reaction $A + B \xrightarrow{k}$ product is first order with respect to $A$ and zero order with respect to $B$. If $a_0$ and $a_t$ are the concentrations of $A$ at $t = 0$ and after time $t \, sec$ respectively,then select the correct relationship -

Which one of the following formulas represents a first-order reaction?

$N_{2}O_{5(g)} \rightarrow 2NO_{2(g)} + \frac{1}{2}O_{2(g)}$
In the above first order reaction,the initial concentration of $N_{2}O_{5}$ is $2.40 \times 10^{-2} \ mol \ L^{-1}$ at $318 \ K$. The concentration of $N_{2}O_{5}$ after $1 \ hour$ was $1.60 \times 10^{-2} \ mol \ L^{-1}$. The rate constant of the reaction at $318 \ K$ is $..... \times 10^{-3} \ min^{-1}$. (Nearest integer)
[Given: $\log 3 = 0.477, \log 5 = 0.699$]

$A$ first-order reaction takes $30$ minutes for $75\%$ decomposition. Calculate its rate constant. (in $\text{ min}^{-1}$)

At $300 \ K$,a gaseous reaction $A(g) \longrightarrow B(g) + C(g)$ follows first-order kinetics. Starting with pure $A$,the total pressure at the end of $20 \ min$ is $100 \ mm \ of \ Hg$. The total pressure after the completion of the reaction is $180 \ mm \ of \ Hg$. The partial pressure of $A$ at $20 \ min$ (in $mm \ of \ Hg$) is: