Answer
(a) increase
(b) I
Work Step by Step
(a) We know that $\Delta S=\frac{\Delta Q}{T}$. This equation shows that the entropy is directly proportional to the heat added. Thus, to maintain the constant temperature, heat is added to the gas and as a result its entropy increases.
(b) The best explanation is option (I) -- that is, heat must be added to the gas to maintain a constant temperature and this increases the entropy of the gas.
