Do you really need to store each line, or just the line the user asks for?
By the way, a C-style string is an array itself (actually a null terminated character array), but I will just refer to it as a string. If you only need one line, you don't need a separate array of strings. You can just loop through the input file a line at a time until you reach the line that the user asked for.
This is one way to get a line from a file into a C-style string and then output the string:
If you can understand that code, then you should be able to use it in your program.
char currentSentence; // Maximum size of line is 100 characters.
input.getline(currentSentence, 100); // Get either 99 characters + null, or the entire line, whichever is first.
cout << currentSentence << endl; // Output the string
If you do want to store every line as an array of sentences, you will want to use an array of strings. In this case, that would be an array of character arrays. That is more complicated, so I won't give you details on that unless you really need to do it.
And as always, if you can use the standard string class (std::string) you would probably have an easier time getting this to work.