Option (C) Long wave terrestrial radiation is correct. According to the textbook context, the Earth receives insolation in short wave form which heats its surface. The Earth, after being heated, becomes a radiating body and radiates energy to the atmosphere in long wave form. This long wave radiation is absorbed by atmospheric gases, particularly carbon dioxide and other greenhouse gases. The context explicitly states that 'the atmosphere is indirectly heated by the earth's radiation' and that 'This energy heats up the atmosphere from below.' Therefore, the atmosphere is mainly heated by long wave terrestrial radiation, not directly by short wave solar radiation.