Explanation:

Let's solve the expression step-by-step:

- Original Expression: 18 - [6 - {13 - (78 - 8 ÷ 2 × 5 of 3)}]

- Step 1: "of" means multiply. So, calculate 5 of 3 = 5 × 3 = 15.

- Step 2: Within the parentheses (78 - 8 ÷ 2 × 15), we handle division and multiplication first.

- 8 ÷ 2 = 4

- 4 × 15 = 60

- Step 3: Continue solving the subtraction inside parentheses.

- 78 - 60 = 18

- Step 4: Substitute back into the expression:

- 18 - [6 - {13 - 18}]

- 13 - 18 = -5

- Step 5: Calculate further:

- 6 - (-5) = 6 + 5 = 11

- Step 6: Finally, solve:

- 18 - 11 = 7

The correct answer is:

- Option 2: 7

.