The correct answer is option 'D' - Assam.

Assam does not have a tribal population that constitutes more than three-fourth of the state population. Let's analyze the tribal population in each of the given states to understand why Assam is the correct answer.

a) Meghalaya:
Meghalaya is a state in Northeast India and is predominantly inhabited by tribal communities. The state has a significant tribal population, with the Khasi, Garo, and Jaintia tribes being the major ones. In fact, tribal population in Meghalaya accounts for more than three-fourth of the state's total population. Therefore, Meghalaya does not meet the condition mentioned in the question.

b) Nagaland:
Nagaland is another state in Northeast India known for its vibrant tribal culture. The state is primarily inhabited by various Naga tribes, and the tribal population in Nagaland is well above three-fourth of the total population. Hence, Nagaland also does not fit the criteria mentioned in the question.

c) Mizoram:
Mizoram, like the other states mentioned, is located in Northeast India. It is home to several tribes, including the Mizo, Hmar, and Lushai tribes. The tribal population in Mizoram constitutes a significant portion of the state's total population and exceeds three-fourth. Therefore, Mizoram does not satisfy the condition given in the question.

d) Assam:
Assam is a state in Northeast India with a diverse population comprising various ethnic groups and communities. While Assam does have a substantial tribal population, it does not exceed three-fourth of the state's total population. Assam is known for its ethnic diversity, with tribes like Bodo, Karbi, Mishing, and Dimasa being prominent. However, their combined population does not constitute more than three-fourth of the state's population. Hence, Assam is the correct answer as it does not meet the condition specified in the question.

In conclusion, among the given options, Assam is the only state where the tribal population does not exceed three-fourth of the state population.