= Solution
\b[B is <oxidative addition>; D is <alkyne> insertion into Ni–aryl followed by carbonyl-oxygen coordination; E is carbonyl insertion into Ni–alkenyl.] The distinction is visible from the bonds made and from consistent <electron counting>.
For B the depicted Ni(0) starting complex has two phosphine and two MeCN donors, giving $10+4+4=18$ <electrons>. Addition across the aryl C–Br bond produces two X-type ligands and Ni(II). The drawn product has lost the two <solvent> donors, giving $8+4+2+2=16$ <electrons>. Intrinsic <oxidative addition> to the unsolvated 14-electron $\mathrm{Ni^0(dppe)}$ fragment raises its count to 16; the overall drawn $18\to16$ change also includes loss of four solvent-donor <electrons>. \b[A falling displayed count does not turn this <oxidative addition> into a <reductive elimination>.]
Before D, the aryl–<alkyne> intermediate is Ni(I), $d^9$, with $9+4_{\rm P,P}+2_{\rm aryl}+2_{\rm alkyne}=17$ <electrons>. Syn migration couples aryl <carbon> to one <alkyne> <carbon> and leaves a Ni–alkenyl bond. Insertion alone removes the separate two-electron <alkyne> donor and would give a 15-electron alkenyl fragment. The ester carbonyl <oxygen> then binds, as drawn, restoring the count to $9+4+2_{\rm alkenyl}+2_{\rm O}=17$. Ni remains $+1$.
In E the Ni-bound alkenyl <carbon> attacks the coordinated ester carbonyl <carbon>, forming the new ring-closing C–C bond. The C=O pi <electrons> move to <oxygen>, and an Ni–O <alkoxide> bond replaces Ni–C. This is carbonyl <migratory insertion>, equivalently intramolecular addition to a coordinated carbonyl. A neutral O donor plus an alkenyl X ligand become one <alkoxide> X ligand, so the drawn count falls \b[$17\to15$], while Ni remains \b[$+1$]. A solvating molecule could fill the resulting vacancy, but it is not shown and should not be silently added to the stated count.
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