The Same-Origin Policy (SOP) is a fundamental security mechanism in web browsers that restricts how a document or script loaded from one origin can interact with resources from another origin. An origin is defined by the combination of protocol, host, and port. Two pages have the same origin if their protocol (e.g., http://), host (e.g., www.example.com), and port (default is 80 for HTTP) match exactly. The given URL is http://www.example.com/dir/page2.html, which uses HTTP, host www.example.com, and the default port 80.

Option 1 ('http://www.example.com/dir/other.html'): Matches the protocol (http), host (www.example.com), and port (default 80), so it is in the same origin.

Option 2 ('http://www.example.com:81/dir/other.html'): Differs in port (81 vs. default 80), so it is a different origin.

Option 3 ('http://www.example.com/dir/other.html'): Identical to Option 1, matching protocol, host, and port, so it is in the same origin. However, since it's a duplicate of Option 1 in the list, it doesn't add a new page.

Option 4 ('http://en.example.com/dir/other.html'): Differs in host (en.example.com vs. www.example.com), so it is a different origin.

Since the question asks which pages are in the same origin, and only Option 1 (and its duplicate Option 3) matches, the correct answer is A ('1 Only'), considering unique pages. The CAP syllabus covers SOP under 'Client-Side Security' and 'Cross-Origin Resource Sharing (CORS).'