Which is an example of a spontaneous exergonic reaction?

It would be B. Marshmallows catching fire because The proteins synthesis needs energy to occur and does not give any form of energy in return, so do photosynthesis. On the contrary, when marshmallows catch fire, they burn and the burning sugar produces heat and energy : this is an exergonic reaction.

For exmaple, more generally, the terms exergonic and endergonic relate to free energy change in any process. Not just chemical reactions. An example of an exergonic reaction is ceullar respiration