This is the 8409th most frequent Russian word.
нк
The Russian term "нк" does not inherently carry meaning on its own and might be an abbreviation, acronym, or context-specific shorthand.
Here, 'nk' is specified as a term, and the sentence identifies its application or lack thereof within the book.
В этой книге нк не используется.
In this book, 'nk' is not used.
The phrase 'по нк' signifies that 'nk' is the subject or topic of the test, acting as a field of study or knowledge.
У нас был тест по нк вчера.
We had a test on 'nk' yesterday.
'Я изучаю нк' means that 'nk' is the subject being studied, treated as a topic or area of learning.