Select the most appropriate option that can substitute the underlined segment in the given sentence.
The boy watched a sci-fi movie in the lieu of playing in the game zone.

The correct answer is in lieu of playing.

Explanation:

The correct idiomatic phrase is "in lieu of", which means "instead of" or "in place of". It is followed by a gerund (verb + -ing) when replacing an action.

Let's analyze the given options:

1. in lieu off playing: Incorrect because "off" is wrong; the correct preposition is "of".

2. in lieu of play: Incorrect because after the preposition "of", the verb should be in its gerund form ("playing") to function as the object of the preposition.

3. on lieu of playing: Incorrect because the idiom uses the preposition "in", not "on".

4. in lieu of playing: Correct standard idiom and grammatical structure.

Therefore, the sentence correctly reads: "The boy watched a sci-fi movie in lieu of playing in the game zone."