Answer:
Reacting hydrogen and oxygen gases to make water
Explanation:
Exothermic means that energy is released into the environment, so let's go through the process of elimination.
Allowing meat to thaw after taking it out of the freezer - NO it is endothermic, and takes energy/heat from the environment to thaw.
Popsicle melting on a warm summer day - NO also endothermic, the opposite of exothermic.
Boiling water in a beaker to convert it to steam - NO because we need to add heat, so it doesn't release any energy.
Rolling a ball up a hill - NO, for a ball to roll up a hill, it needs energy from something else, so it's not exothermic.
Reacting hydrogen and oxygen gases to make water - YES this reaction makes new bonds, which releases energy, so this is exothermic.
