Explanation:

- Option 1: Sikkim

Sikkim is located in the northeastern part of India, known for its mountainous terrain. However, it isn't where the world's tallest girder rail bridge is being constructed.

- Option 2: Manipur

The world's tallest girder rail bridge is being constructed in Manipur. The bridge is part of the Jiribam-Tupul-Imphal rail project, aiming to enhance connectivity in the northeastern region.

- Option 3: Himachal Pradesh

Himachal Pradesh is another mountainous state in northern India but is not the location for this specific rail bridge project.

- Option 4: Jammu and Kashmir

This region is also home to significant rail projects, including the Chenab Bridge, but not the tallest girder rail bridge.

- Option 5: Uttar Pradesh

Known for its plains rather than its mountains, Uttar Pradesh is not relevant to the construction of the tallest girder rail bridge.

In conclusion, the correct answer is Manipur. It is an exciting infrastructure project that will significantly improve transportation in the region.