Which of the following best describes a single-replacement reaction?
a. a compound breaks apart into separate elements.
b. one element takes the place of another in a compound.
c. two elements switch places in two compounds
d. two elements combine to form a compound.

B. One element takes the place of another in a compound.

Explanation:

Single replacement reaction is a chemical reaction in which more reactive element displaces the less reactive element from its salt solution.

Example:

Thus magnesium when dropped into an aqueous solution of releases hydrogen gas by replacing hydrogen from its position.

A. A compound breaks apart into separate elements: Decomposition

Example: Double displacement reaction is one in which exchange of ions take place.  

B. One element takes the place of another in a compound: Single replacement

C. Two elements switch places in two compounds: Double displacement reaction

Example:

D.  Two elements combine to form a compound: Synthesis reaction

Example:  

it is the most commonly accepted one