The correct answer is: d) Nepal.
Bihar is a state in eastern India. It is bordered by Uttar Pradesh to the west, Jharkhand to the south, West Bengal to the east, and Nepal to the north.
Uttar Pradesh is a state in northern India. It is the most populous state in India, with a population of over 200 million people. It is bordered by Bihar to the west, Jharkhand to the south, Madhya Pradesh to the south-west, Rajasthan to the west, Uttarakhand to the north-west, Himachal Pradesh to the north, and Nepal to the north-east.
West Bengal is a state in eastern India. It is the third most populous state in India, with a population of over 90 million people. It is bordered by Bihar to the north, Jharkhand to the south-west, Odisha to the south, Bangladesh to the east, and the Bay of Bengal to the south-east.
Sikkim is a state in northeastern India. It is the least populous state in India, with a population of over 600,000 people. It is bordered by Bhutan to the north, China to the east, Nepal to the west, and West Bengal to the south.