= Solution
For a neutral <borane> $\mathrm{B_nH_m}$ with one terminal B–H bond assigned to each vertex, the skeletal <electron> number is $3n+m-2n=n+m$; an overall charge $q-$ adds $q$ <electrons>. Suitable twelve-electron correspondences are
$$
\boxed{\mathrm C=B_4H_8,\qquad D=[B_3H_8]^- .}
$$
For C, four BH contributions plus four additional <hydrogens> give $8+4=12$ <electrons>. For D, three BH contributions plus five additional <hydrogens> and the negative charge give $6+5+1=12$. They map to the tetrahedral and triangular low-vertex skeletons respectively. The neutral formal alternative for D is $\mathrm{B_3H_9}$, also with twelve skeletal <electrons>.
The <electron counting> correspondence alone must not be read as evidence that the neutral <boranes> are isolable stable bulk materials. Low-vertex hydrogen-rich <boranes> can be transient; \b[the triangular octahydrotriborate anion is the established practical three-boron example], whereas neutral $\mathrm{B_3H_9}$ is a formal analogy rather than an equally established preparative product. The requested skeletal correspondence should be distinguished from a claim about comparable stability or isolation conditions.<Reactions of the octahydrotriborate(1-) anion, B3H8-, with some complexes of cobalt(I), cobalt(II), rhodium(I), and iridium(I), and the characterization of the borallyl complex IrIII(eta3-B3H7)(CO)H(PPh3)2 by Norman N. Greenwood and others>[1980, pp.196–200, reactions of the preparatively accessible octahydrotriborate anion]{ref}
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