Cosmic rays interact with nuclei in the earth's atmosphere and produce...well, this is a bit of a trick question. In their initial interactions with atmospheric nuclei, cosmic rays produce kaons, pions, and nuclei with fewer nucleons (a). Kaons and pions are elementary particles with very short lifetimes. Not long after being produced they decay into gamma rays, muons and neutrinos (b).
The muons and neutrinos make it to the the earth, but the gamma rays initiate pair production of positrons and electrons (c).
You may have wondered what happened to the nuclei with fewer nucleons. They bump into another nucleus in the atmosphere and this process starts all over again, until there are no more nucleons left in the nucleus (i.e. no nucleus). Thus, you get a series of particle cascades as a cosmic ray makes its way through the earth's atmosphere. A diagram of a particle shower is shown below.
Cosmic rays (d) do not produce nuclear fusion upon in their interactions with the earth's atmosphere. Nuclear fusion takes place in extremely hot regions (the centers of stars, etc.) and releases large amounts of energy, but has nothing to do with cosmic rays interacting with the earth's atmosphere.
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