Aggregate

using System.Linq;

public static class Raindrops
{
    private static readonly (int, string)[]  drips = { (3, "Pling"), (5, "Plang"), (7, "Plong") };

    public static string Convert(int number)
    {
        var drops = drips.Aggregate("", (acc, drop) => number % drop.Item1 == 0 ? acc + drop.Item2 : acc);
        return drops.Length > 0 ? drops : number.ToString();
    }
}

• This solution begins by defining an array of tuples. Each tuple has an int and a string.
• The LINQ method Aggregate is called on the drips array. It passes the accumulator, which starts as an empty string, and each tuple to a lambda expression. The lambda expression uses a ternary operator to test if number is evenly divisible by the int in the tuple. If so, it returns the accumulator string concatenated with the string in the tuple. If number is not evenly divisble by the int in the tuple, it simply returns the accumulator string.
• The result of all the iterations of Aggregate is the value of the accumulator string. That value is set to the drops string.
• A ternary operator is used to test the drops string. If the length of drops is greater than 0, then drops is returned from the function. Otherwise number.ToString() is returned from the function.