Consider the following statements:1.The Earth's magnetic field has reversed every few hundred thousand years.2.When the Earth was created more than 4000 million years ago, there was 54% oxygen and no carbon dioxide.3.When living organisms originated, they modified the early atmosphere of the Earth.Which of the statements given above is/are correct?

1. A

   1 only

   (a) 1 only. Geomagnetic reversals every few hundred thousand years are recorded in seafloor stripes and lava, so statement 1 is true, but statement 3 is also true: early life altered the atmosphere, notably by oxygenic photosynthesis. Stopping at 1 is incomplete. This option is not the key.

2. B

   2 and 3 only

   (b) 2 and 3 only. Statement 2 is false: the early Earth did not start with 54% oxygen and zero carbon dioxide; the Hadean–Archean air was rich in CO2 and other volcanic gases, with free oxygen scarce. Statement 3 is true, but it cannot rescue 2. This option is not the key.

3. C

   1 and 3 only

   (c) 1 and 3 only. Magnetic polarity has flipped on a 10^5-year scale. When life appeared it changed the air—most famously the Great Oxidation. Statement 2’s 54% oxygen / no CO2 picture is geochemically wrong. Hence 1 and 3. This option is the official key.

4. D

   1, 2 and 3

   (d) 1, 2 and 3. Statement 2 is incorrect, so all three cannot be right. This option is not the key.

Summary. Official key is (c) 1 and 3 only. Palaeomagnetism shows reversals on timescales of a few hundred thousand years. Living organisms, especially cyanobacteria, transformed the early atmosphere by adding oxygen and drawing down carbon gases. The claim that the newborn Earth already had 54% oxygen and no carbon dioxide contradicts that history. Statement 2 therefore drops out, leaving 1 and 3.