If the initial concentration of a reactant is $a$,how much time will it take for a $100\%$ zero-order reaction to complete?

For a zero order reaction:

Which of the following graphs most appropriately represents a zero order reaction?

Which of the following is the slope of the graph of $[A]_{t}$ versus time for a zero order reaction?

The reaction $2X \to B$ is a zeroth order reaction. If the initial concentration of $X$ is $0.2 \ M$,the half-life is $6 \ h$. When the initial concentration of $X$ is $0.5 \ M$,the time required to reach its final concentration of $0.2 \ M$ will be ........ $hr$.

Which of the following is correct with respect to the graph given?
$[R] = \text{Concentration at time } 't'$
$[R]_0 = \text{Initial concentration}$