Step 1: Understanding the Concept:
"Who" and "whom" are both pronouns used to ask about a person, but they play different grammatical roles. "Who" works as a subject, "whom" works as an object.
Step 2: Key Formula or Approach:
A quick test: try answering the question with "he" or "him" in your head. If "he" fits, use "who". If "him" fits, use "whom".
Step 3: Detailed Explanation:
First sentence: "___ shall I say is calling?" Answer it: "He is calling", not "Him is calling." Since "he" fits, the blank takes "who".
Second sentence: "___ do you advice the most?" Answer it: "I advice him the most", not "I advice he the most." Since "him" fits, the blank takes "whom".
Running this test rules out any option that repeats the same word twice, and it rules out swapping the pair the wrong way round.
Step 4: Final Answer:
The correct words are "who" for the first blank and "whom" for the second, so option (d) is correct.