"Which of the following is suggested in the passage?"
vs.
"Which of the following can be inferred from the passage?"
You are correct that these two questions are very similar in meaning. For the most part, I think you can think of them as asking the same thing. But if you want to get technical, there is a slight difference in meaning.
Something that is suggested is sort of hinted at. You can't really be sure if that's what the author intended, but it seems like that's what they are trying to say. Here's an example:
"It was nice meeting you, John," Julie said, with a twinkle in her eye. "In case you're wondering, I like Italian food and dancing."
The suggestion here is that Julie would like John to ask her out. She's hinting that to him by saying what kind of date she might like. But that idea is only suggested in the passage. It could be that she's just trying to make a new friend and help him get to know her. We don't really know. We're just guessing.
Alternatively, the word inference is often also used to mean a sort of educated guess, but can have a more specific meaning. The third and fourth definitions of "infer" on Dictionary.com are more related to guessing, but the first definition is, "to derive by reasoning; conclude or judge from premises or evidence." In other words, an inference is similar to a suggestion in that you have to figure out something that isn't directly said. But whereas a suggestion truly only hints at something, you can be more sure about an inference. Here's an example:
Julie watched the weather report as she packed a lunch and got ready to go to work. She grabbed her rain jacket and umbrella as she headed out the door.
In this example, it isn't directly said that the weather person reported a chance of rain, but it's a pretty safe bet that they did, since Julie grabbed her rain jacket and umbrella. So an inference is more than a suggestion, but less than coming right out and saying something directly.
I hope this helps!
Reference: https://www.dictionary.com/browse/infer
