A contents of a file are “I love computerrn”. The file is read using fgets() and the result is stored in array buff. What does buff contain?

Chances are that the question is not that: the file content is more likely to be "I love computer\r\n" which is very different.

But this is your homework: so it's your task.
Start by looking at the definition of fgets: fgets - C++ Reference[^] and it's obvious that what you get will depend on the parameters you pass to it - the count is significant. If the count is 3, you will get four characters stored in the buffer: 'I', ' ', 'l', and '\0' - the last being a null terminator to mark the end of the string. Other values will give you other results...

Try it: look at the question, set up the file, and write the code to do it, and use the debugger to find out exactly what happens. This isn't complicated!