Final answer:
To join the given sentences using 'who', the combined sentence can be: "Sam is the person who asked Sita why she was late to school this morning." This integrates both elements of the conversation.
Step-by-step explanation:
To join the sentences using who, we can rephrase the conversation into a single sentence that maintains the original meaning. Sam's question is about Sita's lateness to school, which can be integrated into the sentence structure with who.
For example, the combined sentence could be: "Sam is the person who asked Sita why she was late to school this morning."
This sentence maintains the information that Sam is the one asking the question, as well as the content of the question regarding Sita's tardiness.