Step 1: The question is pure recall of vitamin nomenclature, so eliminate options by assigning each its correct vitamin.
Step 2: Ascorbic acid is vitamin C, so it is not a B vitamin and is dropped. Riboflavin carries the \( B_2 \) label and pyridoxine carries the \( B_6 \) label, so both are removed.
Step 3: The remaining and correct match is thiamine for vitamin \( B_1 \); its water solubility and its role in preventing beri-beri are the standard clues.
Step 4: Hence option (ii), thiamine, is the chemical name of vitamin \( B_1 \).