Knowledge is correct. The plural of knowledge is also knowledge because knowledge is an uncountable noun.
An uncountable noun is a noun that is usually difficult to count, measure, or quantify. Think about hair. It would be very difficult to count hair so it is an uncountable noun. Knowledge is also very difficult to count.
We use knowledge in the following ways:
I have a lot of knowledge about tennis.
There is a lot of collective knowledge on the internet.
We do not use “a” with knowledge
I have knowledge about computers
I have a knowledge about computers.
We often use some/a lot of/any with knowledge
I have a lot of knowledge about coffee.
I have some knowledge about coffee.
I don’t have any knowledge about coffee.
Knowledge of or about?
You can use both knowledge of or knowledge about.
“Knowledge of” is generally when you are talking about the existence of something.
I have no knowledge of stem cell research. (you imply that you barely understand the existence of stem cell research).
“Knowledge about” is generally used and might imply that you have some knowledge about that topic.
I have some knowledge about stem cell research.
In reality “of” and “about” have the same meaning but there is some nuance because of how “of” and “about” are used in English.
An idea of
The existence of
When we use about it is generally to discuss a bit more and of generally means the existence of something.