Question

Which is the correct way to punctuate the salutation of a business letter?

Answer 1

The answer is b.) To whom it may concern:

You need the colon at the end after concern.

Answer 2

The correct way to punctuate is seen in b:
 To whom it may concern:

In formal settings and when you don't know the exact identity of the person a dresses it is usual to use a colon in this situation.