Which of the following is a single replacement reaction? a. cao + h2o yields ca(oh)2 b. mg + 2hno3yields mg(no3)2 + h2 c. naoh + hcl yields h2o + nacl d. ch4 + 2o2yields co2 + 2h2o

The correct option is B. 
Single replacement reaction is a type of chemical reaction, in which one element is substituted for another element in a compound. There are usually two reactants, one is a pure element and the other one is an aqueous compound. The products formed is usually made up of another pure element and a new compound. In the option B, you will notice that, in the reactant part, Mg is a pure element while in the product side, H2 is a pure element.