To solve this question, the user needs to know about JSP and the syntax for defining custom tags in JSP using a tag library descriptor file. Specifically, the user should be familiar with the elements used in the tag library descriptor file.

In this case, the JSP file is using a custom tag defined in a tag library descriptor file. The myTag element in the JSP file should be defined in the tag library descriptor file using the "name" attribute in the "tag" element.

Now, let's go through each option and determine which one is correct:

A. tagname: This is not a valid option for defining a custom tag in a tag library descriptor file.

B. name: This is the correct answer. The "name" attribute in the "tag" element is used to define the name of the custom tag being defined in the tag library descriptor file.

C. tag: The "tag" element is used to define a custom tag in the tag library descriptor file, but it does not specify the name of the tag.

D. prefix: The "prefix" attribute in the "taglib" element is used to specify the prefix used for the custom tag library in the JSP file, but it does not define the custom tag itself.

Therefore, the correct answer is:

The Answer is: B. name