The correct answer is B. This special MIME content type is used to
define a segment which
contains sub-segments representing multiple versions of the same
content. For instance, a
multipart/alternative segment may have two segments within it of types
text/plain and
text/html. It is then left to the e-mail client to choose the most
appropriate format and display
that to the user. As a general rule of thumb, it is always a good idea
to put the easiest to read
(i.e. plain text) versions first in the event that the mail is read
from a non MIME-compatible
e-mail client.