Laura C. answered 05/20/22
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3.The ring that Piers bought his wife is a fake.
The ring Piers bought his wife is a fake.
4. The boy whom Mervyn declared the winner had cheated.
The boy Mervyn declared the winner had cheated.
5. His mother didn't like the man that he had become.
His mother didn't like the man he had become
In #5, the first version is problematic because "that" is not used to refer to individuals (such as "the man"). The other two, #3 and #4 - both versions are acceptable, because "that" can refer to objects (such as "the ring") and "who/whom" can refer to people (such as "the boy").