Correct Answer:
C. on
The correct preposition to use with the verb 'rely' is 'on'. The phrasal verb 'rely on' means to depend on someone or something. This is a common idiom in English, indicating trust or dependence.
- 'Rely at' is grammatically incorrect in this context.
- 'Rely in' is also incorrect; 'in' is used with 'believe in' or 'trust in' but not 'rely'.
- 'Rely of' is not a standard English construction.
Understanding common phrasal verbs and their associated prepositions is crucial for correct English usage, making 'on' the only accurate choice here.