Explanation:

Here’s the thing: you need something that makes sense in real life, not just in grammar books. Let’s break it down:

- "I was not even alert that my friend was ill."

Doesn’t really work. “Alert” is more about being on your toes or watching out for something, not knowing information.

- "I was not even brave that my friend was ill."

That one’s out. Bravery is about facing fear, not about awareness.

- "I was not even aware that my friend was ill."

This makes perfect sense. If you didn’t know your friend was unwell, you weren’t aware of it.

- "I was not even cruel that my friend was ill."

No. Cruelty is unrelated and doesn’t fit the message here.

So, aware is absolutely the right pick. It means you simply didn’t know.