The correct answer is: Only I is implicit.
The statement says that the civic authority has decided to shift all the factories located inside the city limits outside to reduce the level of environmental pollution in the city. This implies that the pollution level in the city is high and that the factories are a major source of pollution. It also implies that the civic authority believes that shifting the factories outside the city limits will reduce the level of pollution.
Assumption I is that the pollution level in the city in future may reduce after these factories are shifted outside the city limit. This is a logical assumption based on the statement. If the factories are a major source of pollution, then shifting them outside the city limits should reduce the level of pollution.
Assumption II is that enough usable land is available outside the city limit for these factories. This is not an implicit assumption. The statement does not say anything about the availability of land outside the city limits. It is possible that there is no land available outside the city limits or that the land available is not suitable for factories.
Assumption III is that many of these factories may shift to some other smaller towns to remain profitable. This is also not an implicit assumption. The statement does not say anything about the profitability of the factories or whether they will be able to remain profitable if they are shifted outside the city limits. It is possible that the factories will not be able to remain profitable if they are shifted outside the city limits and that they will have to shut down.
Therefore, the only implicit assumption is I.