Q.Which of the following is not a valid variable name in Python? Justify reason for it not being a valid name.

The key idea is Python variable naming rules: a variable name must start with a letter (a–z, A–Z) or an underscore (_), followed by letters, digits, or underscores. It cannot begin with a digit.

• (i) 5Radius — starts with a digit, which violates the rule.
• (ii) Radius_ — starts with a letter, followed by a letter and underscore; valid. …

The core rule here is simple: Python follows the same naming convention as most programming languages — a variable name must begin with either a letter (a–z, A–Z) or an underscore (_). It cannot start with a digit. After the first character, you can use letters, digits, or underscores freely.

Why this rule? It prevents ambiguity. If 5Radius were allowed, the interpreter wouldn't know whether 5 is part of a number or a variable name. For example, 5Radius + 3 could be read as 5 (a number) plus Radius (a variable) plus 3, or as a single variable 5Radius plus 3. To avoid this confusion, the language simply forbids names starting with a digit.

Let's check each option:

1. 5Radius — Starts with the digit 5. This violates the rule. Python will raise a SyntaxError if you try to use it.

   Watch out

   A common mistake is thinking that digits anywhere in a name are fine — they are, but not at the start. Radius5 is perfectly valid; 5Radius is not.

2. Radius_ — Starts with a letter (R), followed by letters and an underscore. Allowed. Perfectly valid. …