The two ways of relating what a person has said are 1)direct and 2)indirect (reported speech).In direct speech, the original speaker's exact words are repeated. In indirect/reported speech, the exact meaning is expressed but not necessarily using the original speaker's exact words. Pronouns and possessive adjectives usually change from first or second person to the third person except when the speaker is reporting his own words. Sometimes a noun can be inserted to avoid doubt.

When we turn direct questions and exclamations into indirect speech, they are converted to affirmative statement .( ? and ! is not used).

Option D is correct as when we convert any suggestion or wish (Let's indicates a wish) to reported speech, to + infinitive construction is used, to which is used to join the two dialogues, OK is reported by the adverb positively, present tense is changed to past tense (wished, was fine).

 The other options are wrong as let's, and , and OK can't be used.

The correct answer is D)Sam wished to meet at 8 o'clock to which Sue replied positively and said that it was fine.