The correct answer is option 4, which states that the characteristic of a virus that best denotes its unique nature is its ability to replicate and invade the human body.

Let`s dissect each option to understand it better:

Option 1 states that a virus can attack bacteria. While it is true that some viruses can infect and attack bacteria, this is not an exclusive characteristic of viruses and is not unique to them.

Option 2 states that a virus can be crystallized. While it is possible to crystallize some viruses under certain conditions, this is also not a unique characteristic of viruses.

Option 3 states that a virus can replicate and invade the human body. This is a correct characteristic of viruses. Viruses are microscopic infectious agents that require a host to replicate and can invade the cells of various organisms, including humans.

Therefore, option 4 is the correct answer. It correctly identifies the unique nature of viruses in their ability to replicate and invade the human body.