In Java programming, which of the following is a valid declaration of a String variable?

Introduction / Context:
Java treats java.lang.String as a reference type. A declaration must follow Java’s literal and casting rules. This question checks your understanding of valid String initializations and the difference between character literals, Unicode escapes, and reference nulls.

Given Data / Assumptions:

• We are declaring String references in standard Java.
• Single quotes denote a single char literal, not a String.
• Unicode escapes like \uXXXX represent a single char code point when used inside character or string literals.

Concept / Approach:
A String variable may be assigned either a String object (e.g., a double-quoted literal) or the null reference. Casting a char to String is not legal without calling an API (such as Character.toString or String.valueOf). Single quotes are for char, not String.

Step-by-Step Solution:

Verification / Alternative check:
Compile the snippets individually; only option A compiles without extra helper methods.

Why Other Options Are Wrong:
They either misuse single quotes (char literal) or perform an invalid cast from char to String.

Common Pitfalls:
Confusing ' ' (char) with "" "" (String), and assuming casts can magically convert primitives to reference types.

Final Answer:
String s1 = null;