Select the correct indirect form of the given sentence.
“Let us go to the hills for a vacation,” Mother said to us.

Correct option is D

The correct indirect form of the given sentence is (d) Mother suggested that we should go to the hills for a vacation.
Direct speech- Direct speech, also known as quoted speech or direct quotation, is a way of reporting someone's exact words.
Indirect speech- Indirect speech, also known as reported speech or indirect quotation, is a way of reporting someone's words or thoughts indirectly, without using their exact words.
Indirect Speech is supported by some verbs like requested, ordered, suggested and advised. Forbid-forbade is used for negative sentences. Therefore,  the Infinitive in indirect speech changes into the imperative mood in the direct speech.
The sentence is a suggestion, so 'suggested that we should' correctly conveys the original meaning.