During a flight from Delhi to Tokyo the following are the landing airports : I. Hong Kong II. Hanoi III. Taipei IV Bangkok The correct sequence of the landing at these airports during an onward journey is

On an eastward (onward) flight from Delhi to Tokyo the intermediate airports are encountered in order of increasing longitude. The eastward sequence is: Bangkok, Hanoi, Hong Kong, Taipei — i.e., IV, II, I, III, so option 2. Delhi is an international origin point referenced among Indian international airports [1]. Bangkok is a major Asia–Pacific aviation/meeting hub (regional APAC/ICAO context) consistent with being encountered first among the listed stops east of Delhi [2]. Major Asian hub stops such as Hong Kong and Taipei are commonly used on eastbound routings between South Asia and East Asia, supporting their placement later in the sequence. The ordering above follows their progressively more easterly longitudes.

Sources

1. [1] Geography of India ,Majid Husain, (McGrawHill 9th ed.) > Chapter 12: Transport, Communications and Trade > Air Sewa > p. 31
2. [2] https://www.icao.int/sites/default/files/APAC/Meetings/APANPIRG/2018%20APANPIRG%2029/0-Report/Final-Report_APANPIRG-29.pdf