の中(なか)でDentro de (A), (De) entre (A), De (entre) (A), En (A)

～のうち(で) and のなかで are similar expressions that both mean 'among', but their usage can differ. ～のうち(で) is most often used with number + counter phrases. For example, 5人のうち4人が合格した ('Out of 5 people, 4 passed.'). With non-numerical groupings, like スポーツの中でどれが一番好き？ ('Among sports, which do you like best?'), のなかで is more common, especially in speech.

のなかで～がいちばん～ se usa para decir que algo es 'lo mejor' o 'lo más (A)' dentro de un grupo o conjunto de opciones. Por su parte, のなかで se suele traducir como 'en' o 'de entre' un grupo o conjunto de opciones. Incluso sin ir acompañado de いちばん, puede transmitir la idea de preferencia, como en 果物の中で何が好き？ ('¿Qué fruta te gusta más?' o '¿Qué fruta prefieres?').

のなかで and 同士 can both refer to groups or relationships, but their meanings and usage differ. のなかで means 'among' and is used to highlight one or more members within a group, e.g. サッカー選手の中で誰が好き？ ('Among soccer players, who do you like?'). 同士, on the other hand, emphasizes mutual relationships, e.g. サッカー選手同士で話し合った means 'They talked as fellow soccer players.'.

Though both contain なか, their meanings are different. なかなか is often used in negatives to express difficulty, like 'not easily' (なかなかできません), or in positives to mean 'quite' (なかなかいい). In contrast, のなかで means 'among' and is used when referring to one or more members of a group. For example, 果物の中でりんごが一番好き means 'Among fruits, I like apples the best.'.