Final answer:
The word 'profound' in the given sentence means having a severe or intensely deep impact, which suggests that the environmental effect of border walls is significant.
Step-by-step explanation:
In the sentence 'For example, in some cases, the environmental impact of border walls has been profound.' the word 'profound' means having a very great or intensely deep impact. In the context used, it suggests that the environmental impact is severe and significant. It does not mean difficult, clever, or heartfelt, which are the other options provided for the definition. Hence, the correct definition of 'profound' in this sentence is 'severe'.