(i) 2,3-dibromobutane / $\ce{CH3CHBrCHBrCH3}$ ✔

(ii) curly arrow from the C=C bond to the nearer Br atom of $\ce{Br2}$ AND curly arrow from the Br-Br bond to the far Br atom «heterolytic fission of the polarized $\ce{Br^{\delta+}-Br^{\delta-}}$

carbocation $\ce{CH3-CHBr-C^{+}H-CH3}$

curly arrow from a lone pair / the negative charge on $\ce{Br-}$ to the $\ce{C+}$ ✔

(iii) the C=C double bond has a high electron density «π electrons» which attracts electrophiles «and polarizes $\ce{Br2}$» / the π bond can break so that two new bonds form ✔

(iv) $\ce{C_{n}H_{2n}}$ ✔

(v) London (dispersion) forces «only» act between the alkane molecules ✔

these forces become stronger as the chain length / number of electrons / size of the electron cloud / molar mass increases, so more energy is needed to separate the molecules ✔

(i) [1] for the name or the structure. (ii) [1] per marking point; the arrows must start from the π bond, from the Br-Br bond and from the bromide lone pair respectively. (iii) [1]; accept «susceptible to electrophilic attack». (iv) [1]; subscripts not essential. (v) M1: London (dispersion) forces, not «van der Waals forces» alone; M2: strength increasing with chain length, number of electrons or surface area.