Sentence that needs correction : "A dilemma is when a person is faced with two or more choices, each of which leads to undesirable consequences.
Why is the use of when wrong? Why should when be ommitted?
Sentence that needs correction : "A dilemma is when a person is faced with two or more choices, each of which leads to undesirable consequences.
Why is the use of when wrong? Why should when be ommitted?
Although this sentence is perfectly fine in casual speech it is not quite as correct as possible. "When" specifies a point in time, while a dilemma refers to a situation.
A situation in which a difficult choice has to be made between two or more alternatives, especially equally undesirable ones http://www.oxforddictionaries.com/us/definition/american_english/dilemma
The correct answer would be: "A dilemma is a situation in which a person is faced with two or more choices [...]"
The question obviously was produced from this definition.
As some comments have said, that sentence is fine in casual speech.
In formal speech, the problem is with "is when" not just "when." The word "is" expects to be followed by a noun or an adjective, like "A dilemma is a kind of problem." On the other hand, "when," expects to say when something happens, like "The ground is wet when it is raining."
Replacing "is" with "occurs" would be fine: "A dilemma occurs when a person is faced..."
You should not simply omit "when" in this case, because "a dilemma is a person..." immediately causes a problem: A dilemma is not a kind of person.