The only valid answer to that question as phrased is the US government service which issued the visa, which in the vast majority of cases would be “US embassy in X” or “US consulate in Y”.
This is clearly redundant with the next question “Issued in city”, but anything else would be factually incorrect.
You may want to contact them to check that this is indeed what they are asking for, but there is no other possible interpretation of the question as it is written.
PS: actually there could be another interpretation which would be the name of the officer who issued the visa, but I’m not sure that info is actually given, and I seriously doubt it could be of any use.