To display hyperlinks without an underline, you can use the CSS property "text-decoration" and set it to "none". This will remove the underline from the hyperlink text.

Option D is the correct answer:

The Answer is: D

To create a list that displays its items with squares, you need to use CSS. Specifically, you can use the list-style-type property to specify the style of the bullet point or marker for each item in the list.

Now, let's go through each option and explain why it is right or wrong:

A. type: square: This option is incorrect because the CSS property for specifying the list style is list-style-type, not type. Additionally, the value for a square marker is square, so the correct syntax would be list-style-type: square.

B. list-style-type: square: This option is correct. By using the list-style-type property and setting its value to square, you can create a list that displays its items with square markers.

C. list-type: square: This option is incorrect because there is no CSS property called list-type. The correct property to use is list-style-type.

D. style-list: square: This option is incorrect because there is no CSS property called style-list. The correct property to use is list-style-type.

The Answer is: B

The correct HTML for referring to an external style sheet is:

A.

Explanation:

Option A is the correct answer. The `tag is used to link an external resource in HTML. In this case, it is used to link an external style sheet. Therelattribute specifies the relationship between the current document and the linked resource, and in this case, it is set to "stylesheet". Thetypeattribute specifies the MIME type of the linked resource, and in this case, it is set to "text/css", indicating that it is a CSS file. Thehref` attribute specifies the path to the external style sheet file, in this case, "mainstyle.css".

Option B is incorrect because the `` tag is used to define an embedded style sheet within the HTML document itself, not to link an external style sheet.

Option C is incorrect because there is no <stylesheet> tag in HTML. The correct tag to define a style sheet is <link>.

Option D is incorrect because the url attribute does not exist for the <link> tag. The correct attribute for specifying the URL of the linked resource is href.

Therefore, the correct answer is A. <link rel="stylesheet" type="text/css" href="mainstyle.css">