= Solution
\b[Frequency-dependent ac susceptibility] probes relaxation: a peak in the out-of-phase component occurs approximately when $\omega\tau=1$. Measuring its displacement with frequency determines $\tau(T)$; an activated region may fit $\tau=\tau_0\exp(U_{\rm eff}/k_BT)$. A low-temperature departure can reveal tunnelling or other relaxation channels.
\b[Low-temperature, sweep-rate-dependent <magnetization> <hysteresis>] tests whether reversal is slow on the measurement timescale. A loop opening below the blocking temperature, variation with sweep rate and possible tunnelling steps provide complementary evidence. Isothermal remanent <magnetization> decay after field removal directly measures the associated relaxation time. A single open loop alone would not distinguish isolated-molecule blocking from collective ordering or impurities.
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