Learn English – One of these most intelligent students whose example (is/are) being

Your base sentence is itself ambiguous.

1. It may mean that each of the tasks with which you are concerned outputs a single new task.

2. It may mean that each of the tasks with which you are concerned outputs multiple new tasks.

3. It may mean that the tasks with which you are concerned participate in an aggregation or collaboration which outputs multiple new tasks.

4. It may mean that the tasks with which you are concerned participate in multiple aggregations which each output one or more tasks.

Context will ordinarily sort out which of these is intended, so in practice most people will find all three sentences you put forward acceptable (provided you replace the singular verb outputs in the third one with output).

But if you are concerned to avoid the consequences of the Adamantine Law^† you will compose a different sentence for each. Given your preference for expressions with whose rather than which, I suggest:

1. Generally speaking, this concerns tasks each of whose output is another task.

2. Generally speaking, this concerns tasks each of whose outputs are more tasks.

3. Generally speaking, this concerns a set of tasks whose aggregate output is more tasks.

4. Generally speaking, this concerns tasks participating in aggregations each of whose output is one or more tasks.

Myself, I prefer to replace the expression each of whose with whose each. However, a discussion of whose each on our Elder Sister Site suggests that this expression is unfamiliar to many contemporary readers; so it should probably be avoided under the Insularity Principle^‡.

^† The Adamantine Law: “Any expression that can be misunderstood will be.”

^‡ The Insularity Principle: “Any expression, however well established, will be rejected as ‘ungrammatical’ by those unfamiliar with it.”

It is grammatically incorrect, but most people think it is natural and don't care.

The simple subject is thing, which is singular, so the verb must be singular to agree with it. Notice that in the subordinate clause, scares is singular, agreeing with thing.

The speaker got confused because it seems strange to say that a singular thing "is" a plural like oranges. That's more than one orange, right? Perhaps the speaker could revise the sentence so the subject is "the things that scare me the most". But there are two problems with that. First, the idiomatic phrase is "the thing that scares me the most". If you say "the things that scare me the most", that's weaker, because then you aren't pointing out the #1 scariest thing. Second, the speaker starts saying the sentence before choosing the main verb. By the time the speaker has said "The thing that scares me the most", it's too late to change it. At that point, the speaker sees oranges coming up ahead, and scrambles to choose the verb. "Is" sounds wrong because of oranges; "are" sounds wrong because of thing. Oh no!! There's no time to think this through, so the speaker compromises and chooses a verb to agree with oranges, dimly sensing that something is wrong.

And then life goes on.