Solution
We are asked what word would fit at the beginning of the second sentence.
Each - Wouldn't fit because students being able to use their own language wouldn't make for ALL the important themes, which is what the word "Each" would imply here.
Someone - Makes no sense, "someone" would refer to a person, but we are talking about themes here.
Some - This would be coherent with the given sentences and would connect them best, as students being able to use their own language to understand better would make for SOME of the themes.
One - This is wrong, because the sentence says "are to ensure." We can't use "one" and "are" together, as "one" is singular. It would be correct if the full sentence was "One of the important themes IS to ensure..."