The best answer is A. Pregnant women should not exercise at very high altitudes.
This statement is true as high altitude can pose risks due to lower oxygen levels, which could affect both the mother and the baby.
Option B is false because moderate exercise is generally encouraged during pregnancy unless there are specific medical concerns.
Option C is misleading; while pregnant women are advised to engage in regular physical activity, the recommendation is usually for about 150 minutes of moderate-intensity exercise per week, not necessarily vigorous exercise.
Option D is also false; lying in a supine position for extended periods is generally discouraged during the later stages of pregnancy due to potential complications.
Thus, option A is the most accurate statement.