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Hello,
The tip is to substitute, for example in the first question, $\displaystyle R_p$ by x.
If $\displaystyle \sqrt{v_{esc}}=kx$, where k is a constant, then it's a line.
For the fourth one, substitute $\displaystyle x=\frac{1}{R_p}$ and see if $\displaystyle v_{esc}=k' \ x$, where k' is also a constant.