The correct answer is: Only conclusion I follows.
The statement says that those who undertake physical exercise for at least half an hour a day are less prone to have any heart ailments. This does not mean that people who do not undertake physical exercise are definitely prone to heart ailments. It is possible that there are other factors that contribute to heart ailments, such as diet, stress, and genetics. Therefore, conclusion II does not follow.
However, the statement does suggest that moderate level of physical exercise is beneficial for health. This is because the statement says that those who undertake physical exercise for at least half an hour a day are less prone to have any heart ailments. Therefore, conclusion I follows.
It is important to note that the statement does not say that moderate level of physical exercise is the only factor that contributes to a healthy life. It is possible that there are other factors that are also important, such as diet, stress, and genetics.