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Ca2+ is wrong. An inhibitory postsynaptic potential is where the action of a NTM at a postsynaptic receptor results in hyperpolarisation of the Vm of the postsynaptic neuron. Hyperpolarisation is where the Vm of the cell becomes more negative than resting potential which can be generalised as approx -70mV for neurons. The electrochemical gradient of Ca2+ in a cell at resting potential is directed into the cell, so if the membrane became more permeable to Ca2+ the Vm would become less negative, not more negative. The answer is K+, though it can also be Cl-. The electrochemical gradient of K+ is directed from the inside of the cell to the outside of the cell at resting potential and K+ of course has a positive charge. Increase in potassium conductance results in an increase in positive charge efflux from the cell which effectively mean that the cell loses positive charge. Losing positive charge is electrically the same as gaining negative charge, and thus the Vm becomes more negative, giving you your hyperpolarisation and your IPSP. Note that if one of the options was Cl-, that would also be correct. The electrochemical gradient of Cl- is directed into the cell at resting Vm, and it is negative, so an increase in Cl- conductance would mean an increase in negativity, a hyperpolarisation, and thus an IPSP.
Serotonin and GABA are wrong because they are neurotransmitters, not ions. Na+ and Ca2+ are wrong because at resting Vm, an increase in Na+ or Ca2+ conductance would result in depolarisation (EPSP), not hyperpolarisation (IPSP).
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