Which of these nine alkanes are gases at room temperature (20°c)

methane, ethane, propane and butane are the alkanes that are gases at room temperature. all four of these alkanes have four or fewer carbon atoms. alkanes with more carbon content are liquid or solid at room temperature. hexane and octane are examples of alkanes that are liquid at room temperature.

radioactive decay occurs in unstable atomic nuclei – that is, ones that don't have enough binding energy to hold

the nucleus together due to an excess of either protons or neutrons.

there is none because earth is stationary and flat.