Which of the following cations are stabilised by resonance (delocalisation of a lone pair or a pi bond into the empty $p$ orbital)?

Methoxymethyl has an oxygen lone pair, allyl has a C=C, and benzyl has an aromatic ring adjacent to the cationic carbon, so all three show resonance. The tert-butyl cation has only sigma C-H and C-C bonds next to the cation, so it is stabilised by hyperconjugation and not resonance.