Correct Answer: D
Explanation:
When Reported Speech begins with ‘Let’ in sense of a suggestion in Direct Speech, then Reporting verb ‘said’ in Direct Speech → Reporting verb ‘proposed’/‘suggested’ in Indirect Speech
Conversion Rule is Sub + propose/suggest + to + object + that + Sub + should + V₁ (1st from) + …… .
Hence options ‘D’ fits in the context.