They are all very similar. The choice depends on what you want to find out and what sort of relationship has already been established between the company and the person being asked. For me the real choice is between (2) and (3).
"How did you hear about us?" (3) is good but to me suggests a first encounter. Perhaps how you would ask someone who is visiting a web site for the first time.
"How did you learn about us?" (2) is good. Has the same meaning as with "hear" but also suggests the person being asked has done more research or spent more time using the product/service/whatever.
"How did you get / become familiar with us?" and "How did you get to know about us?" seem too wordy. I have struggled with trying to articulate my dislike for these two options. It boils down to a feeling that they convey the wrong ideas. Perhaps it is that "familiar" suggests friendship or family rather than a business relationship. Perhaps it is that "get to know" also suggests friendship.
PS. I think you are asking about a "questionnaire" rather than a "questionary".