= Solution
{c}
The photon energy fixes the resonant <cyclotron frequency> by $\hbar\omega=575\,\mu\mathrm{eV}$. Consequently
$$
\frac{m_{\mathrm{cr}}}{m_e}=\frac{e\hbar B}{m_e(575\,\mu\mathrm{eV})}\simeq0.2013\,\frac{B}{\mathrm T}.
$$
Read the two absorption maxima as approximately $B_L=0.95$--$0.97\,\mathrm T$ and $B_H=2.10\,\mathrm T$. For $\mathbf B\parallel[001]$, the two valleys whose long axis is along $z$ have $m_{\mathrm{cr}}=m_1$, whereas the other four have $m_{\mathrm{cr}}=\sqrt{m_1m_2}$. Hence
$$
\boxed{m_1\simeq0.19m_e,\qquad m_2=\frac{[0.2013B_H/\mathrm T]^2}{m_1/m_e}m_e\simeq0.92\text{--}0.94m_e.}
$$
For the representative reading $B_L=0.96\,\mathrm T$, the values are $m_1=0.193m_e$ and $m_2=0.925m_e$. The graph does not warrant additional precision. The larger longitudinal <effective mass> means flatter dispersion and a longer constant-energy ellipsoid along the valley axis.
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