C++ string literals: select the single option that is an <em>invalid</em> string constant (i.e., not a valid string literal) while others are valid strings.

Introduction / Context:
In C and C++, a string constant (string literal) is a sequence of characters enclosed in double quotes. It produces an array of characters terminated by a null character. This question asks you to identify which presented token is not a valid string literal, ensuring clarity about the difference between character and string constants.

Given Data / Assumptions:

• Valid string literals use double quotes, for example, "text".
• Character literals use single quotes and represent a single code unit (e.g., 'A').
• Options include both quoted forms and content with digits and spaces.

Concept / Approach:

• Anything in double quotes is a string literal, regardless of its textual content (digits, words, spaces).
• Single quotes denote a character literal; more than one character between single quotes is not a string literal and is invalid as a character literal in strictly conforming code (multi-character literals are implementation-defined integer constants, not strings).
• Therefore, an entry surrounded by single quotes containing multiple characters is not a valid string constant.

Step-by-Step Solution:

Verification / Alternative check:

Why Other Options Are Wrong:

• "Hello, world!", "7.15 pm", "i like e", "1234e12": All are proper string literals because they use double quotes.

Common Pitfalls:

• Confusing the lexical form (single vs double quotes) and assuming content determines type.
• Misusing single quotes for words; only double quotes produce strings.

Final Answer: