I am a student studying in USA and I am from Nepal. I have a student visa to study in USA. I will be going back to Nepal this December and return in January but I am planning to stay in S.Korea for about 1 week before coming back to USA. I believe my best option would be to have transit in Incheon Airport and make the stay a week long. Am I eligible for a transit Visa(B-2-1)?
According to this link (under the "Transit tourists bound for another country" category), the following persons are eligible for visa-free travel to Korea:
Persons holding a visa (re-entry permit, permanent residency, etc.) to enter the U.S. (including Guam and Saipan), Canada, Australia, or New Zealand (hereinafter referred to as "the four advanced countries") ....
1) who transit through Korea to go to one of the four advanced countries.
2) who stay in one of the four advanced countries and take a direct flight from the country to Korea to go to a country of origin or a third country.
The only requirement aside from those above is that
Must have a confirmed onward flight ticket for departure within 30 days after entering Korea, in addition to having no record of criminal offence in the five countries mentioned above.
Basically, it sounds like you're allowed to stay in Korea VISA-free as long as you're coming from or going to the U.S., you have your flight out of Korea booked, and you're not staying for more than 30 days. In the case where you are coming from the U.S., the last leg of your flight must be from U.S., Canada, Australia, or New Zealand.
Here is another link that confirms the same information. (Click on Short term visit --> Tourist/Transit (General))
Here is another person who asked a similar question and received a nearly identical response.
As stated in Timatic, the database used by Airlines:
Visa required, except for Nationals of Nepal with a visa issued by [...] USA [...] if:
holding confirmed onward tickets on flights departing within 30 days; when arriving from a third country and departing to the country that issued the visa (e.g. DEL-ICN-YVR)
So, because you have a US visa, you do not need another visa to visit Seoul for max 30 days