Making one line string

Though there are many ways to do this. However, here is a succinct solution (to ô̖p̖̝̃̉en̺͔͍͌̍̈́ y̯͇̦͌̍͌oũ̠͍̫̀͡r̜̚ m̠̞͑̍i̭̓ṇ̼̍͘d͈̪͍̑̚͝) using a Dictionary and LINQ.

Since the words are separated by a space, then a simple string.Split would work fine.

var dict = new Dictionary<string, string>(StringComparer.InvariantCultureIgnoreCase)
{
   {"Grr", "Lion"},
   {"Rawr", "Tiger"},
   {"Ssss", "Snake"},
   {"Chirp", "Bird"},
};

var animals = Console
   .ReadLine()?
   .Split(' ', StringSplitOptions.RemoveEmptyEntries)
   .Select(word => !dict.TryGetValue(word, out var animal) ? "N/A" : animal)
   .ToList();

if (animals?.Any() == true)
   Console.WriteLine(string.Join(", ", animals));
else
   Console.WriteLine("Nothing entered");

Input

grr bob chirp ssss

Output

Lion, N/A, Bird, Snake

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Additional Resources

String.Split Method

Returns a string array that contains the substrings in this instance that are delimited by elements of a specified string or Unicode character array.

Dictionary<TKey,TValue>(IEqualityComparer)

The IEqualityComparer<T> implementation to use when comparing keys, or null to use the default EqualityComparer<T> for the type of the key.

Dictionary<TKey,TValue>.TryGetValue(TKey, TValue) Method

Gets the value associated with the specified key.

Enumerable.Select Method

Projects each element of a sequence into a new form.

string.Join

Concatenates the elements of a specified array or the members of a collection, using the specified separator between each element or member.

Null-conditional operators ?. and ?[]

Available in C# 6 and later, a null-conditional operator applies a member access, ?., or element access, ?[], operation to its operand only if that operand evaluates to non-null; otherwise, it returns null

Language Integrated Query (LINQ)

Language-Integrated Query (LINQ) is the name for a set of technologies based on the integration of query capabilities directly into the C# language. Traditionally, queries against data are expressed as simple strings without type checking at compile time or IntelliSense support. Furthermore, you have to learn a different query language for each type of data source: SQL databases, XML documents, various Web services, and so on. With LINQ, a query is a first-class language construct, just like classes, methods, events. You write queries against strongly typed collections of objects by using language keywords and familiar operators.