C) Tc-99 is a radioactive isotope also known as radioisoptope.
The melting point of potassium = 
Melting point of titanium = 
Titanium has a stronger metallic bonding compared to potassium. Titanium being a transition metal has greater number of valence electrons (4 valence electrons) contributing to the valence electron sea compared to potassium which has only one valence electron. The atomic size of Titanium much lower than that of potassium, so the bonding between Titanium atoms is stronger than that of potassium. Hence, the melting point of Titanium is much higher than that of potassium.
Endothermic reactions, on the other hand, absorb heat and/or light from their surroundings. For example, decomposition reactions are usually endothermic. In endothermic reactions, the products have more enthalpy than the reactants. Thus, an endothermic reaction is said to have a positive<span> enthalpy of reaction. This means that the energy required to break the bonds in the reactants is more than the energy released when new bonds form in the products; in other words, the reaction requires energy to proceed.</span>