In English, there are a few nouns that are always plural, like 'pants,' 'glasses,' etc. These are typically considered singular in other languages. So, while 'The pants are red,' in English, 'Die Hose ist rot' in German.
It's been answered already, but I just like to think that "Hose" translates more literally to "pair of pants".
You are correct that der Jungen would be the plural and since it says des Jungen we know it's one boy's pants/trousers that seem to be missing.
Weak nouns are similar to strong nouns, they're just declined differently. There is no way to tell a weak noun from a strong nouns - you just have to memorize it.
There are some Masculine nouns in German which receive N ending in akkustaiv, dativ and genetiv. This is one of them.
Did anyone else have to translate this exact sentence into German on an earlier question and get marked wrong for not using 'Hosen'?
Yep! Love that it's been reported multiple times over many months, the reverse (i.e. this sentence) is fine, and it's still not fixed.
There's no problem. Both "die Hose" (one pair of pants) and "die Hosen" (multiple pairs of pants) are accepted, but you need to make sure your subject-verb agreement is correct as well. The only correct translations are "Wo ist die Hose des Jungen?" and "Wo sind die Hosen des Jugen?". "Hose" takes a singular verb (ist) because it's singular, and "Hosen" takes a plural verb (sind) because it's plural.
I already knew that. To be clearer: I was marked wrong for saying "wo ist die Hose" instead of using the plural "wo sind die Hosen" on the reverse (it's kinda confusing that there's different comment threads for each, but I guess that helps with clutter). Maybe I made some other small mistake and the "you got THIS wrong!" thing picked up on my not using the singular instead of the actual mistake, which happens pretty frequently. Maybe I imagined the whole thing...
The pronunciation of Jungen is really hard to understand from the guy's voice
If it were plural, it'd be "der Jungen", not "des Jungen".
Jungen is an annoyingly special word called a weak noun or an -N noun. These mostly arbitrary (Masculine only) words get an -n ending in Accusative, Dative, and Genetive cases just to make your learning experience a little more fun :P