I don't know if this is just me, but I'd consider this a pretty badly worded question. For models of population dynamics, the rate at which the size of a population changes over time is normally accepted to be the net balance of four fundamental processes: Birth, Immigration, Death, and Emigration ('BIDE'; see for example here). Birth and immigration increase a population, death and emigration decrease it. Unless you exterminate a species then it's going to be pretty difficult to assert that a population decrease from either source is 'permanent' - new individuals are likely to be born into the local population and/or immigrate eventually.
An example that specifically refutes the assertion that 'emigration results in a permanent decline in population size' would be a source-sink metapopulation, in which emigration repeatedly/constantly occurs from the 'source' population but it doesn't permanently decrease in size.
As a lesser quibble, although 'migration' can be taken to refer to temporary movement/resettling, it is pretty common to consider immigration and emigration to each be components of migration.
So I would certainly argue that for the question as asked, 'D' is at least as valid an answer as 'C', but in both cases it is impossible to say a permanent decrease would occur. At the same time, if it was changed to 'could occur', you could make a case that 'A' could be a valid answer (since 'migration' includes emigration).