Its worldwide popularity is mainly due to its theme music, which showcases ABBA's timeless songs in a fresh and vital way 24 retains the essence of both pop music and good musical theater.
答案：(D) that
關係代名詞 that 引導形容詞子句修飾 way，作子句主詞；how（不能這樣接在名詞way後面，how本身已含「the way」的意思不能重複）、what（需要先行詞的關係代名詞是that/which不是what）、where（不是地方）都不合。
空格前面是 way，很多人立刻想到「the way how」這個組合，就挑了 how。也有人看到前面已經有一個 which，覺得同一句不能再出現關係詞，於是改挑 what。
先看空格後面：retains 是一個動詞，而它前面沒有主詞。所以空格必須是一個「自己兼任主詞」的關係代名詞，這一步就能刷掉 how 和 where——它們是副詞性的，本身不能當主詞。再找先行詞：a fresh and vital way（一種清新而有活力的方式）。是這個「方式」保留了流行音樂與優質音樂劇場的精髓，所以先行詞是 way，關係代名詞用 that（也可以用 which）當子句的主詞。還原成兩句驗算：It showcases ABBA’s timeless songs in a fresh and vital way. 加上 The way retains the essence of both pop music and good musical theater. 順帶把 the way 的規矩弄清楚：the way 後面可以接 that、可以接 in which，也可以什麼都不接，但不能寫成 the way how——因為 how 本身就含有 the way 的意思，兩個放在一起是重複（劍橋 English Grammar Today）。所以 how 在這裡不是「可以但不好」，是不合文法。
how：不能接在 way 後面，理由如上。what：what 本身已經包含先行詞，等於 the thing that；這裡前面已經有先行詞 way，不能再用 what。where：關係副詞，用在地方，而且它不能當子句的主詞，後面必須接完整的句子。
the way that（…的方式）、the way in which（…的方式）、the essence of（…的精髓）、be mainly due to（主要歸因於…）、showcase something（展現、呈現某物）、timeless songs（歷久不衰的歌曲）。
On March 23, 1999, the musical MAMMA MIA! made its first public appearance in London. It 21 the kind of welcome it has been getting ever since. The audience went wild. They were literally out of their seats and singing and dancing in the aisles.
MAMMA MIA! has become a 22 entertainment phenomenon. More than 30 million people all over the world have fallen in love with the characters, the story and the music. The musical has been performed in more than nine languages, with more productions than any 23 musical.
Its worldwide popularity is mainly due to its theme music, which showcases ABBA's timeless songs in a fresh and vital way 24 retains the essence of both pop music and good musical theater. It has 25 so many people that a film version was also made. To no one's surprise, it has enjoyed similar popularity.