Translation:These girls are famous.
That's one way to see it, but it makes more sense to see 名 as part of 名声 (めいせい) meaning fame or reputation.
Which again can be seen as "your name's voice", what people "hear" when they hear your name aka your reputation.
Are fame and dream connected culturally, like I suspect tears and wave are, or is this just a funny similarity like bow and sea, plum and rain?
What's wrong with "these are famous girls"? I just got marked incorrect for it
Because この cannot be used as a stand alone demonstrative. It HAS to modify a noun - These girls indicates that you understand that. If you wanted to say "these are..." you'd use a stand alone demonstrative and make that your subject for the sentence ie. これらは also ゆうめい would be modifying the noun 有名 な 女の子たち - famous girls.
It's a suffix that makes people and animals plural. In this instance it changes girl to girls.
女の子たち means "girls", just like 私たち means "we"/"us". The たち part makes it plural.
Because the 'ko' in onna no ko means child. So they are talking about female children.