The answer is D, Lithium. The first two options are SSRIs and the third is an SNRI, none of which are first line for bipolar disorder (BPD) maintenance. Lithium remains a treatment option with demonstrated efficacy for stabilization of mood symptoms and prevention of suicide. While recent studies are unable to show consistently statistically significant differences between Lithium and other maintenance options for BPD for suicide prevention, it has been known for some time that lithium is effective in suicide prevention vs placebo. SSRIs in general should not be used in BPD as they may unmask manic episodes (although they are sometimes used as supplements during depressive episodes). Lithium is thought to prevent suicide through several mechanisms-- reduction in mood symptoms and reduction in impulsivity being among the most accepted theories. A large cohort study published in June 2016 demonstrated that lithium was effective in reducing unintentional injury and intentional self harm vs several other maintenance therapy agents, adding weight to the theory that lithium reduces impulsivity in addition to its mood stabilizing properties. In general, primary maintenance therapy for BPD may consist of lithium, certain anticonvulsants, and several second generation antipsychotics.

Publication reference : Lithium in the prevention of suicide in mood disorders: updated systematic review and meta-analysis. BMJ. 2013 Jun 27;346:f3646. doi: 10.1136/bmj.f3646. Review. PubMed PMID: 23814104.
Reference URL: http://www.bmj.com.libproxy.tulane.edu:2048/cgi/pmidlookup?view=long&pmid=23814104