Which best describes the way photographs are considered by historians? primary source documents that are useful in the study of history reliable sources for the study of history only if from a named source secondary source documents that are useful in the study of history unreliable sources for the study of history because they are too subjective useful sources for the study of personal histories, but not for general history ?

So, here are the options:
primary source documents that are useful in the study of history - this is the best answer!

 reliable sources for the study of history only if from a named source - I would say it's best if you know the author but it can be very useful also without it

secondary source documents that are useful in the study of history - no, they are primary (=from the time) documents

unreliable sources for the study of history because they are too subjective- they can present a subjective side of an event, but they can't easily fake a whole event - they are still reliable

 useful sources for the study of personal histories, but not for general history - theymight be more useful for personal histories, but are also very usefuf for general histories