Cameras that automatically focus using phase detection have been aroun

let us look at the answer options -

A - cannot be inferred. all the author says is this - "Cameras that automatically focus using phase detection have been around since the 1970s." this statement implies that they are still around.

B - correct answer. read the argument carefully. "these early auto-focusing cameras were slow and easily confused by low light and low contrast. In the 1980s, Japanese manufacturers introduced automatically focusing cameras that remedied these problems"
"some camera manufacturers" refers to "Japanese manufacturers".

C - Incorrect. "early auto-focusing cameras were slow and easily confused by low light and low contrast." - This only implies that they did not function very well in low light and contrast. this does not mean that they could not determine which of the subjects needed to be in focus.

D - Incorrect. "professional 35 mm cameras have the ability to automatically focus and to quickly determine which of several possible subjects needs to be most in focus." - this only means that they do a better job than previous cameras. Does not mean that they will never be confused by low light or contrast.

E - incorrect. "professional 35 mm cameras have the ability to automatically focus and to quickly determine which of several possible subjects needs to be most in focus." - all we can infer is that cameras today do a better job at focusing than previous cameras did.
Does not mention that today's auto focus cameras are better than today's cameras that do not have auto focus. Note that the argument talks only about auto focus cameras. Information about non auto focus cameras cannot be inferred.