To answer this question, we need to understand what a keyword is in the context of Java programming.
In Java, keywords are reserved words that have predefined meanings and cannot be used as identifiers (such as variable names or class names). These keywords are used to define the syntax and structure of the Java programming language.
Let's go through each option to understand which one is a valid keyword in Java:
Option A) interface - This option is correct. "interface" is a valid keyword in Java and is used to define a collection of abstract methods that can be implemented by a class.
Option B) string - This option is incorrect. "string" is not a valid keyword in Java. However, "String" (with an uppercase "S") is a valid keyword and is used to represent a sequence of characters.
Option C) Float - This option is incorrect. "Float" is not a valid keyword in Java. However, "float" (with a lowercase "f") is a valid keyword and is used to represent floating-point numbers.
Option D) unsigned - This option is incorrect. "unsigned" is not a valid keyword in Java.
Based on the explanations above, the correct answer is Option A) interface.