Invalid Identifier in Java

An identifier is a name given to a variable, method, class, or other programming elements in Java. It is used to uniquely identify these elements within a program. Identifiers in Java must follow certain rules and conventions.

In the given options, the invalid identifier in Java is:

c) 123abcd

Explanation:

In Java, an identifier must adhere to the following rules:

1. It must start with a letter (a-z or A-Z), underscore (_), or a dollar sign ($).
2. After the first character, it can contain letters, digits, underscores, or dollar signs.
3. It cannot contain spaces or any special characters except underscore (_) or a dollar sign ($).
4. It cannot be a reserved word or keyword.

Let's analyze each option:

a) myVariable
- This is a valid identifier in Java as it starts with a letter and contains only letters.

b) _variable
- This is also a valid identifier in Java as it starts with an underscore and contains only letters.

c) 123abcd
- This is not a valid identifier in Java because it starts with a digit (number) instead of a letter, underscore, or dollar sign.

d) $price
- This is a valid identifier in Java as it starts with a dollar sign and contains only letters.

Conclusion:

The option c) 123abcd is not a valid identifier in Java as it violates the rule that an identifier must start with a letter, underscore, or a dollar sign.