The enamine of cyclohexanone with pyrrolidine is treated with each of the following, then with $\mathrm{H}{}_3\mathrm{O}{}^{+}$. Which give a new C-C bond at the alpha carbon of the ketone?

The enamine beta carbon is a soft nucleophile. It undergoes SN2 with methyl iodide, conjugate (Michael) addition to methyl vinyl ketone and acylation with acetyl chloride. tert-Butyl bromide is a tertiary halide, which undergoes elimination, not SN2, so no C-alkylation occurs.