Mastering Java: The Ultimate Quiz for 'Thinking in Java' 2026 - Free Java Programming Practice Questions and Study Guide

The method toLowerCase() is the only built-in method in Java that changes all letters in a String to lowercase. Option A, tolowerCase(), is incorrect because it includes a typo and is not a valid method name. Option C, lower(), is incorrect because it is not a built-in method in Java. Option D, convertToLower(), is incorrect because it is not a valid method name in Java and there is no built-in method with this name. Therefore, the correct answer is B, toLowerCase().