a) The framework groups Reducer inputs by keys
b) The shuffle and sort phases occur simultaneously i.e. while outputs are being fetched they are merged
c) Since JobConf.setOutputKeyComparatorClass(Class) can be used to control how intermediate keys are grouped, these can be used in conjunction to simulate secondary sort on values
d) All of the mentioned

Answer: d
Explanation: If equivalence rules for keys while grouping the intermediates are different from those for grouping keys before reduction, then one may specify a Comparator.