To answer this question, you need to understand how to specify complex restrictions for the content of an element.
Option A) sequence - This option is incorrect. "Sequence" is a term used in programming languages to specify the order in which statements or elements should be executed, but it does not specifically address complex restrictions for the content of an element.
Option B) regular expression - This option is correct. Regular expressions are a powerful tool used to define patterns for matching strings. They can be used to specify complex restrictions for the content of an element by defining a pattern that the content must match.
Option C) XSD - This option is incorrect. XSD stands for XML Schema Definition, which is a language used to describe the structure and constraints of XML documents. While XSD can be used to specify restrictions for the content of an element, it does not specifically address complex restrictions.
Option D) XLT - This option is incorrect. XLT is not a commonly used term or technology in the context of specifying complex restrictions for the content of an element.
The correct answer is B) regular expression. This option is correct because regular expressions can be used to define patterns that specify complex restrictions for the content of an element.