When we use to convert and when to transform?

[Annaa]

Synonyms Lesson, Advanced Level

ESL/EFL Test #105 "Synonyms for exchange", question 8

Marriage completely ......... his character.

(a) converted
(b) mutated
(c) transformed
(d) shifted

Synonyms Lesson, Advanced Level

ESL/EFL Test #105 "Synonyms for exchange", answer 8

Marriage completely transformed his character.

Correct answer: (c) transformed
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When we use to convert and when to transform?
It's quite confusing for me:(

[Conchita]

In some contexts, convert and transform can be synonyms. 'Convert', however, has the added meaning of turning (someone) from one religion to another.