The construction is “owe something to someone”. The fishermen are people and the relative clause is nonrestrictive, so “to whom” supplies both the required preposition and personal relative pronoun. The completed clause is “to whom the flood victims owed their lives”, C.
More Verbal Aptitude: Grammar and Vocabulary questions
GATE 2019 BT Q5– Until Iran came along, India had never been in kabaddi.
GATE 2019 CY Q1– John Thomas, an writer, passed away in 2018.
GATE 2019 CY Q2– I permitted him to leave, I wouldn’t have had any problem with him being absent, I?
GATE 2019 CY Q5– The minister avoided mentioning the issue of women’s reservation in the private sector. He…
GATE 2019 MT Q1– The fishermen, the flood victims owed their lives, were rewarded by the government.
GATE 2019 MT Q5– Until Iran came along, India had never been in kabaddi.