Bases:
are generally proton acceptors
produce oh− ions in solution
are electron pair donors
may include all of these characteristics

The correct answer is "may include all of these characteristics".

Explanation:

By definition, in chemistry a base is a chemical species that acts in the three concepts included in the answers. Bases are generally proton acceptors when acids act as proton donators; bases releases hydroxide (OH-) ions and positive ions in aqueous solutions; and bases are electron pair donors when acids are electron-pair acceptors.

c) should be the answer.