A. div#button: This is incorrect because it attempts to select a div with the ID button, but the element's actual ID is blueBtn.

D. div.blueBtn: This is incorrect because it uses a class selector (.) to find an element with the class blueBtn. The element's ID is blueBtn, not its class.

1. jQuery API Documentation - ID Selector ("#id"): "Selects a single element with the given id attribute... For id selectors

jQuery uses the JavaScript function document.getElementById()

which is extremely efficient." This source validates #blueBtn (Option B) as a correct and efficient selector.

URL: https://api.jquery.com/id-selector/

2. jQuery API Documentation - Element Selector ("element"): "Selects all elements with the given tag name." This can be combined with other selectors

such as the ID selector

to create more specific selections like div#blueBtn.

URL: https://api.jquery.com/element-selector/

3. W3C - CSS Selectors Level 3 Specification

Section 6.2

ID selectors: The specification defines the # notation for ID selectors (e.g.

#blueBtn) and type selectors (e.g.

div). It also defines how to chain these simple selectors (e.g.

div#blueBtn) to form a more specific sequence. This validates both B and C as syntactically correct according to the underlying CSS selector standard that jQuery uses.

URL: https://www.w3.org/TR/css3-selectors/#id-selectors