Final answer:
The song by Abba that came out in 1975 and was later included in a musical about a young girl getting married is "Mamma Mia!".
Step-by-step explanation:
The song by Abba that came out in 1975 and was later included in a musical about a young girl getting married is "Mamma Mia!". The musical "Mamma Mia!" features the music of Abba and tells the story of a young girl on the eve of her wedding, trying to discover the identity of her father. The song "Mamma Mia" is one of Abba's most popular hits and is a key part of the musical.