We can drag the correct words to the blanks as follows:
Prior to the early 1900s, many people believed that the nuclear family was the ideal family form and that less developed nations had not developed universal notions of romantic love. This belief was challenged by Bronislaw Malinowski's research, which showed that the family was a universal human institution.
How to complete the text
We can complete the text by noting the fact that the nuclear family was the popular family unit in the 1900s.
So, before this time, many thought that this ideal family form had not understood the universal notions of love but the research by Bronislaw indicated that the Trobriand Islanders practiced matrilineal descent, where kinship was traced through the mother's line, and he noted that romantic love was not the primary basis for marriage.