The correct choice is 'がんばろう' (volitional form of 'がんばる' - to do one's best). The grammar pattern '[Volitional Form] + と思う' expresses the speaker's current intention or resolve for the future. Sentence context: '...so I think I will continue to do my best in the future as well.' The other options are: 'がんばりなさい' (imperative command: do your best!), 'がんばらせたい' (want to make someone do their best), and 'がんばれる' (potential: can do one's best), which do not collocate with '〜と思います' to express personal intent.