Rate of change and kinetics order of reaction question
Indeed, the only thing you'll need is solving the first order differential equation (but before that, let me define $R_0 := \ce{[RSH]_0}$ and $C :=\ce{[Ker-S-S-Ker]}$.
Now, let's solve it assuming (that's what is given) that $R_0$ can be considered a constant.
Then, we have : $$\frac{\operatorname{d}C}{\operatorname{d}t} = -kR_0C \implies \frac{\operatorname{d}C}{\operatorname{d}t} + kR_0C = 0 \implies C(t) = \exp\left(-\frac{t}{kR_0}\right) + C_0$$ (integration constants can be determined using initial values, I won't be giving further details here on how to solve this...)
Hence, since the concentration of your molecule is an exponential function of time, it is inevitably a convex function (so only curves C and E can work).
Finally, a simple thought on the values of $k$ and $R_0$ show that curve E cannot be the answer as the decay is to &quot;fast&quot;.
Hope this helps, if needed I can also provide a &quot;variables separation&quot; method of solving the differential equation.