Because the word inference means "a conclusion reached on the basis of evidence and reasoning." This means that when the question asks the process of gathering objective information you would make inferences. Summarizing is when you give/tell the overall idea or the point of a text someone/something. Making observations is almost the same so if this is wrong it is probably D.
The salutation is to greet the person you are addressing, it goes before the introduction. The introduction is a brief summary of the letter itself and the body is the letter itself.
