If a carbon atom is bonded to two hydrogen atoms and two carbon atoms which type of bond must exist between the carbon atoms

Carbon to carbon bond is nonpolar covalent since the difference of their electronegativity’s is 0. Carbon has an electronegativity of 2.5. 2.5 - 2.5 is 0 making the bond between those two atoms nonpolar covalent. I hope I answered your question.

a. energy is absorbed in the reaction.

if in a given reaction the energy of products is greater than the energy of reactants it means that reaction is endothermic and during the reaction, the energy is absorbed by reactants. in an endothermic reaction, the products have more stored chemical energy than the reactants. and in this reaction the reactants are more stable than products before the compounds with less energy are always more stable.