also: このなか
expression
of these; among these; between these
Refers to a choice, number, subset, or comparison within a set already mentioned. このうち is common for a counted set; この中で can also mean “among this group.”
候補は五人います。このうち二人を選びます。
There are five candidates. We will choose two of them.
この中で一番好きな写真を選んでください
Please choose the photo you like best from these.
expression
in here; inside this; in this
Only when written この中 · Only with reading このなか
Used with この中 read このなか for something physically or conceptually inside the thing or space being indicated.
鍵はこの中に入っています。
The key is in here.
A broader noun meaning “inside,” “within,” or “among”; このうち is the demonstrative phrase “among these” or “of these.”
Means “among those” in a similar set-reference use, but it also commonly means “before long” or “sometime soon,” which このうち does not.
More concrete than このうち when it means “inside this”; it can overlap with このうち when referring to members of a group.
A transparent phrase from この (“this”) plus うち/内 (“inside; within; among”). The spelling この中 uses 中, emphasizing “inside” or “within this.”