= Solution
With the <Fourier transform> convention $\phi(\mathbf k)=\int d^3x\,e^{-i\mathbf k\cdot\mathbf x}\phi(\mathbf x)$, scale invariance of the position-space three-point function gives
$$
\langle\phi(\mu\mathbf k_1)\phi(\mu\mathbf k_2)\phi(\mu\mathbf k_3)\rangle
=\mu^{-9}\langle\phi(\mathbf k_1)\phi(\mathbf k_2)\phi(\mathbf k_3)\rangle.
$$
Because $\delta^{(3)}(\mu\sum_i\mathbf k_i)=\mu^{-3}\delta^{(3)}(\sum_i\mathbf k_i)$, the reduced <primordial bispectrum> must satisfy
$$
\boxed{B(\mu k_1,\mu k_2,\mu k_3)=\mu^{-6}B(k_1,k_2,k_3)}.
$$
In the result of part b, the bracket has momentum degree five, while $k_1^3k_2^3k_3^3K^2$ has degree eleven. Its total degree is therefore $-6$, exactly as required.
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