Which of these changes with time for $1^{st}$ order reaction

$\begin{array}{1 1}(i)\;\text{rate of reaction}\\(ii)\;\text{rate constant}\\(iii)\;\text{half life}\\(iv)\;\text{concentration of reactant}\end{array}$

$\begin{array}{1 1}(a)\;(i)only\\(b)\;(iii)only\\(c)\;(i)\; and\;(ii)only\\(d)\;(i)\;and\;(iv)only\end{array}$

For first order reaction, rate constant is constant at constant temperature.
Half life=$\large\frac{ln 2}{K}$
So half life is also constant.
Hence (d) is the correct answer.