Explanation:

- The area of a triangle is calculated as \( \frac{1}{2} \times \text{base} \times \text{height} \).

- If the base is increased by 10%, the new base becomes \( 1.10 \times \text{base} \).

- If the height decreases by 20%, the new height becomes \( 0.80 \times \text{height} \).

- The new area becomes \( \frac{1}{2} \times (1.10 \times \text{base}) \times (0.80 \times \text{height}) \).

- The new area formula simplifies to \( \frac{1}{2} \times \text{base} \times \text{height} \times 1.10 \times 0.80 \).

- The factor affecting the area becomes \( 1.10 \times 0.80 = 0.88 \).

- This means the area reduces to 88% of the original, indicating a percentage decrease of \( 100\% - 88\% = 12\% \).

- The correct answer is Option:4 - 12%.