It is necessary that the string is displayed under number N, which is generated by random.
What function should be used for this? Preferably Blue Functions (CSTDIO)

Answer 1, Authority 100%

You need the function of opening a file, read and closing. Open the file, read the line and increase the counter. When the counter has reached the desired, we display the string.
Schematic code:

char * getline (char * filename, int nedded) {
 Char Line [250];
 File * F = Open (FileName, "R");
 int c = 0;
 While (C & LT; Needed & Amp; & amp; not eof (f)) {
   FSCANF (F, "% S", Line);
   C++;
 }
 Close (F);
 If (C! = NEDDED) {
  RETURN NULL;
 }
 RETURN STRDUP (LINE); // Do not forget then free the buffer
}
char * line = getline ("input.txt", n);
If (Line) {
 PrintF ("% s \ n", line);
 FREE (LINE);
}

Answer 2, Authority 20%

// Indexing rows from zero
INT N_ReadLine (Const Char * Fname, Int N, Char * BUF, INT LEN) {
  Char C;
  File * FP = Fopen (FNAME, "RT");
  if (FP == NULL)
    Return 0;
  While (! FeOF (FP) & amp; & amp; (n & gt; 0)) {
    FSCANF (FP, "% * [^ \ N \ r]% C", & amp; c);
    --n;
  }
  If (! FeOF (FP))
    FGETS (BUF, LEN-1, FP);
  ELSE.
    * buf = '\ 0';
  FClose (FP);
  Return (int) (* buf! = '\ 0');
 }
 INT MAIN (Void) {
  Char BUF [255];
  // count a 2-line
  if (n_readline ("levels.txt", 1, buf, sizeof (buf)))
     Puts (BUF);
  // count the 3rd line
  if (N_ReadLine ("Levels.txt", 2, Buf, Sizeof (BUF)))
     Puts (BUF);
  Return 0;

}

Answer 3

In other answers, options are at clean C, and here you are on C++:

# include & lt; fstream & gt;
STD :: String Get_Line_N (Const Std :: String & Amp; FileName, Size_T n)
{
  STD :: IFStream F (FileName);
  // here would not prevent the verification that everything is in order with the file
  STD :: String Line;
  BOOL NOTRACHED = TRUE;
  While (std :: getline (F, Line) & amp; & amp; notreached = (n-- & gt; 0))
    ;
  if (! notreached)
    Return Line;
  ELSE.
    // there is no such line, report error
    // For example, throw a suitable exception.
    Throw Std :: Range_ERROR ();
}

For comparison, here on C #:

string getlinbynumber (String Filename, Int N)
{
  var line = file.Readlines (FileName) .Skip (n) .firstordefault ();
  if (Line! = NULL)
    Return Line;
  ELSE.
    Throw New IndexOUTOFRANGEEXCEPTION ();
}

Exercise: Why do you need a variable NotReached ? Why not check the value n ?