An alkane reacts with a halogen to form a halocarbon. this is an example of an elimination reaction  a substitution reaction  a condensation reaction  a dehydration reaction

I think it is an example of a substitution reaction. Alkanes are saturated hydrocarbon which means each carbon atom has a maximum number of bonds, they lack double or triple bond. A substitution reaction involves substituting hydrogen atom by another element such as halogens. For example Ethane is the second member of alkane when reacted with a halogen such as chlorine, one chlorine atom replaces one hydrogen atom to form chloroethane and the other bromine combines with hydrogen to form hydrogen bromide.
CH3CH3 + Br2 = CH3CH2Br +HBr

During photosynthesis, plants transform electromagnet energy from the sun into chemical energy.