Fill in the blank with the most appropriate word.
“Open your mouth.” The dentist told me ________________.
a) to open his mouth
b) opened my mouth
c) open your mouth
d) to open my mouth

Hint: Simple present tense gets converted into simple past tense in indirect speech, as it is a reported form of speech. It is not the exact quote someone said. However, to convert an imperative, the verb is changed to an infinitive.

Complete answer:
 In grammar, there are two kinds of speech - Direct and Indirect.
In direct speech. we use quotes to repeat the exact thing an individual said. However, in indirect speech, the direct quote is said in the third person, and so the quotation marks are removed. In order to change the indirect speech into direct speech, we must change the tense of the sentence, and make the necessary punctuation changes.
When we want to report something someone said without speech marks, we use indirect speech. It is also called reported speech.
From direct to indirect, the tense changes from present to past. When you convert the sentence from direct to indirect speech, the tense changes.

Now we know exactly what we are looking for. Let’s look at the given options-
a) to open his mouth - This option contains an infinitive but the possessive in this sentence is wrong and it does not fit into the given sentence. This is not the required answer. So, this is an incorrect option.
b) opened my mouth - This option does not contain an infinitive and it does not fit into the given sentence. This is not the required answer. So, this is an incorrect option.
c) open your mouth - This option also does not contain an infinitive and it does not fit into the given sentence. This is not the required answer. So, this is an incorrect option.
d) to open my mouth - This option has the correct infinitive and it has the correct possessive. This fits into the given sentence. This is the required answer. So, this is the correct option.

Note:
 Identify the grammatically correct and the most contextually sound option. Converting direct speech to indirect speech has to follow certain rules in order to be grammatically correct.