If one horsepower is equal to 746 watts, how much horsepower does a highly trained athlete generate by doing 340 joules of work per second for an hourone

746 joules per second = 746 watts = 1 horsepower

340 joules per second = 340 watts = (340/746) = 0.456 horsepower

Power is a RATE or a SPEED of doing work.  

How long you do it doesn't matter.  

Just like 30 miles per hour doesn't change whether you do it for an hour or for 10 minutes.

There are two types of Reactions,

Endothermic reactions are the type chemical reaction in which the reactants absorb heat energy from the surroundings. The energy of reactants is more than the energy of product

Exothermic reactions are the type of chemical reactions in which release of energy takes place in the form of heat or light. In an exothermic reaction, energy is released. The energy of the products is less than the energy of the reactants.

The key word is "RELEASE". The reaction in which energy is released is the Exothermic reaction thus the Option B that is EXOTHERMIC REACTION is correct.

It is asking that which process stores the energy that means it is asking which process undergoes endothermic reaction. The answer is Option C that is PHOTOSYNTHESIS is correct.

Because, sunlight is being absorbed during the reaction and according to the definition of endothermic reaction if heat is being absorbed it is termed as an endothermic reaction.