The correct answer is D. Mother tongue.
Mother tongue is the best medium of instruction at primary level because it is the language that children are most comfortable with and can express themselves best in. It is also the language that they are most familiar with and have the most exposure to. This makes it easier for them to learn and understand new concepts.
National language and international language are not necessarily the best medium of instruction at primary level. This is because children may not be as comfortable with these languages and may not have as much exposure to them. This can make it difficult for them to learn and understand new concepts.
Regional language can also be a good medium of instruction at primary level. However, it is important to note that the regional language may not be the same as the mother tongue of all children. This means that some children may not be as comfortable with the regional language as they are with their mother tongue.
Overall, mother tongue is the best medium of instruction at primary level. This is because it is the language that children are most comfortable with and can express themselves best in. It is also the language that they are most familiar with and have the most exposure to. This makes it easier for them to learn and understand new concepts.