The correct answer is: B. a few
A few means “a small number of”. It is used to refer to a small number of things or people that are not specified. For example, you might say “I have a few friends” or “I have a few questions”.
Little means “not much”. It is used to refer to a small amount of something. For example, you might say “I have little money” or “I have little time”.
Few and little are often confused, but they have different meanings. Few refers to a small number of things, while little refers to a small amount of something.
In the sentence “Once, I have visited America. I know a few cities there”, the speaker is saying that they have visited a small number of cities in America. They are not specifying how many cities they have visited, but they are saying that it is a small number.
The other options are incorrect because they do not accurately reflect the meaning of the sentence. Option A, “few”, is incorrect because it does not specify that the speaker has visited a small number of cities. Option C, “little”, is incorrect because it does not refer to a small number of cities. Option D, “a little”, is incorrect because it does not accurately reflect the meaning of the sentence.