Explanation:

- Start by solving the expression inside the brackets: $(9-4)$ results in 5.

- Evaluate the expression in the curly braces: $(8÷8\text{of}4)+(4÷4\text{of}2)$.

- Calculate $8\text{of}4$ as $8\times 4=32$. Thus, $8÷32=0.25$.

- Calculate $4\text{of}2$ as $4\times 2=8$. Thus, $4÷8=0.5$.

- Sum $0.25+0.5=0.75$.

- Divide 5 by 0.75 to get $\frac{5}{0.75}=20/3$.

- Multiply 21 by 20/3 to get $21\times \frac{20}{3}=140$.

- Option 1: 140

- Option 2: 80

- Option 3: 100

- Option 4: 20