Final answer:
To target an element by its ID in CSS, you would use the # symbol followed by the id value. For the heading with the ID 'title', you would use '#title {}' in the CSS.
Step-by-step explanation:
To select a heading by its id in CSS, you would use the hash symbol (#) followed by the identifier of the element. In the case of the heading 'Fluffy cats', if the id is 'title', you would write #title in your CSS file to target that specific element.
The correct choice from the options provided would be:
(Choice A) #title { }
The other options are incorrect because Choice B selects elements with 'title' as their tag, which is not valid HTML; Choice C selects all h1 elements, regardless of their id or class; and Choice D would select elements with 'title' as their class, not id.