B
Step-by-Step Solution
Insight: Verify total row and column sums first, then use forced moves starting with the most constrained row or column.
Exam route: Row sums = 3 + 1 + 2 + 2 = 8. Column sums = 2 + 2 + 2 + 2 = 8. Consistency check passed. Row 2 requires exactly 1 shaded cell. Inspecting the options: Option (i) has 2 shaded in Row 2 (invalid). Option (iii) has 2 shaded in Row 2 (invalid). Option (ii) has 1 shaded in Row 2. Verify Option (ii) columns: Col 1 has 2, Col 2 has 2, Col 3 has 2, Col 4 has 2. All constraints satisfied. Option (ii) is correct.
Learning route:
Step 1: Consistency Check. Sum of row requirements (3+1+2+2 = 8) must equal sum of column requirements (2+2+2+2 = 8). They match.
Step 2: Identify Extremes. Row 2 requires only 1 shaded cell out of 4. This is the tightest constraint.
Step 3: Option Elimination. Instead of solving the grid from scratch (which is time-consuming), evaluate the given options against the tightest constraint.
Step 4: Check Row 2 in all options. Option (i) shows 2 shaded cells. Option (iii) shows 2 shaded cells. Both are immediately discarded.
Step 5: Verify the survivor. Option (ii) has exactly 1 shaded cell in Row 2. Now verify its columns: Col 1 (Rows 1, 4) = 2. Col 2 (Rows 2, 4) = 2. Col 3 (Rows 1, 3) = 2. Col 4 (Rows 1, 3) = 2. All column sums are exactly 2. Option (ii) is the unique valid configuration.