The correct answer is $\boxed{\text{B. }10^6 \text{ mho}}$.
Conductance is the inverse of resistance, and is measured in siemens (S). One siemens is equal to one ampere per volt (A/V). One microampere is equal to one millionth of an ampere (μA). Therefore, if a voltage of one volt is applied to a circuit and one microampere of current flows through it, the conductance of the circuit is $10^6 \text{ mho}$.
Option A is incorrect because one nanomho is equal to one billionth of a siemens (μS).
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