Part 2: Root Locus, Transfer Functions and Unit Step Response?
From the root locus plot, the open loop transfer function has a real-valued pole at $s_0=0$ and a complex conjugate pole pair $s_1=-1.5+ 2j$ and $s_1^*$. There's also a zero at $s=-2$. Consequently, $G(s)$ is given by
$$G(s)=\frac{s+2}{s(s-s_1)(s-s_1^*)}$$
The closed-loop transfer function is
$$H(s)=\frac{kG(s)}{1+kG(s)}=\frac{k(s+2)}{s(s-s_1)(s-s_1^*)+k(s+2)}\tag{2}$$
The final value of the step response equals $H(0)=1$, which leaves options $A$ and $B$.
Looking at the root-locus plot, we can see that the real-valued pole is (a bit) closer to the imaginary axis than the complex-conjugate pole pair. So that real-valued pole is (slightly) dominant. Based on this observation one can guess that step response $A$ is correct because it clearly shows the influence of the real-valued pole (i.e., a smooth increase with no overshoot), whereas step response $B$ just looks like a step response of a second-order system with a complex-conjugate pole pair.
In sum, apart from looking at the final value of the step response given by $H(0)$, you can check if there are one or more dominant poles that determine the behavior of the step response. You should be familiar with the step responses of first and second-order systems; this will allow you to make an educated guess also for higher order systems. This website gives a good overview of step responses of first, second, and higher order systems.