Answer:
The option that is correctly punctuated is:
2 The doctor said, "I have been checking a lot of patients since morning. So, I am very tired now."
Step-by-step explanation:
When using direct speech, we must place a comma right before quoting what the person said, commonly after the verb "say". For that reason, we can eliminate options 1 and 4, since they do not have a comma after 'The doctor said.'
We are left with options 2 and 3. Now, we must look at the sentence inside that quotation marks. In number 3, the sentence has no punctuation excerpt for a period at the end. However, when "so" is used as a conjunction, we should at least place a comma before it. For that reason, we should choose number 2.
NOTE: Number 2 does not present the best punctuation for the use of "so" as a subordinating conjunction. If you typed number 3 properly, then the answer is really number 2. However, if you forgot the place a comma when typing number 3 but it actually has it, then the correct option should be number 3. Please, check the original worksheet or site where you found this question.