Which of the following media would be best to make a formal introduction to a potential client?

a. letter

b. email

c. blog

d. all are equal

The key part of the question is that the introduction should be a "formal" one - so a blog is not a good idea.

Also, a letter is definitely a more formal kind of communication than email, so while an email can be an acceptable way of formally introducing oneself, a letter is better.

The answer is option a.