According to Karl Marx, the attainment of communism is possible only after the emergence of an exploitation-free society. Marx believed that the ultimate goal of communism was to establish a classless society, where everyone is treated equally and there is no exploitation of one class by another.

Option 1: The resolution of tension between the leader and led is not a prerequisite for communism according to Marx. In fact, Marx believed that the abolition of class distinctions would lead to the elimination of leaders and the establishment of a society where decisions are made collectively.

Option 2: While Marx did advocate for a revolution to overthrow the capitalist system and establish communism, he did not specify that it had to be a permanent revolution. The revolution was seen as a means to achieve communism, not an end in itself.

Option 4: Marx did acknowledge the existence of differences between urban and rural areas in capitalist societies, but the disappearance of these differences alone does not guarantee the attainment of communism. According to Marx, communism can only be achieved when the fundamental economic structure of society is transformed and exploitation is eliminated.

In conclusion, the correct answer is option 3, as the emergence of an exploitation-free society is considered by Marx as a necessary condition for the attainment of communism.