Which of the following best describes the behavior of a gated channel?
(a) it stays open continuously when stimulated
(b) it opens more frequently in response to a given stimulus
(c) it opens more widely as the stimulus becomes stronger
(D) it remains closed if unstimulated

Option B ,  It opens more frequently in response to a given stimulus.

Explanation:

A gated channel is one which opens specifically to some kind of stimulus.  These stimulus could be a neurotransmission, a variation or change in the pressure, voltage, lights, chemical environment or any other physical parameter.  

For example  

Ligand gated ion channel which have receptors forming a membrane ion channel to allow passage of specific chemicals such as Na+, K+, Ca++, or Cl− only after the activation of receptor/channel complex.  

Hence, option B is correct

an ion is a charged atom or molecule. it is charged because the number of electrons do not equal the number of protons in the atom or molecule. an atom can acquire a positive charge or a negative charge depending on whether the number of electrons in an atom is greater or less then the number of protons in the atom.

need to see circuit to get time constant