Step 1: Understanding the Concept:
Examine the constraints specifically regarding person B's trousers.
Step 2: Identifying the Vector of Change:
- B's Shirt: Red.
- B's Trouser cannot be: Red (Shirt/Trouser same color rule), Blue (Red/Blue combo rule), Cream or Yellow (explicitly stated in clue ii).
Step 3: Calculation:
Available trousers: White, Black, Indigo, Blue, Red, Cream, Yellow.
- Not Red, Not Blue, Not Cream, Not Yellow.
- D already wears Indigo.
- This leaves White or Black.
- Following the logic for E, C, and G's colors, Black is the only remaining valid assignment for B.
Step 4: Final Answer:
B's trousers are Black. Thus, the correct option is (a).