RelevanceIndex.cs

using System;
using System.Linq;

// http://bgcoder.com/Contests/Practice/Index/143#3
class RelevanceIndex
{
    public static void Main(string[] args)
    {
        string keyWord = Console.ReadLine();
        int numberOfParagraphs = int.Parse(Console.ReadLine());
        string[] paragraphs = new string[numberOfParagraphs];
        var punctuation = new string[] { ",", ".", "(", ")", ";", "-", "!", "?", " " };

        for (int i = 0; i < numberOfParagraphs; i++)
        {
            var inputWords =
                Console.ReadLine()
                    .Split(punctuation, StringSplitOptions.RemoveEmptyEntries)
                    .Select(w => w.ToUpper().Equals(keyWord.ToUpper()) ? keyWord.ToUpper() : w);

            paragraphs[i] = string.Join(" ", inputWords);
        }

        var orderedParagraphs = paragraphs.OrderByDescending(p => CountStringOccurrences(p, keyWord.ToUpper()));

        foreach (string orderedParagraph in orderedParagraphs)
        {
            Console.WriteLine(orderedParagraph);
        }
    }

    private static int CountStringOccurrences(string text, string pattern)
    {
        int count = 0;
        int i = 0;
        while ((i = text.IndexOf(pattern, i)) != -1)
        {
            i += pattern.Length;
            count++;
        }
        return count;
    }
}