Solve the operations inside the innermost parenthesis.

First, perform the division:

$$91 \div 7 = 13$$

Next, perform the subtraction within the parenthesis:

$$13 - 12 = 1$$

So, the expression inside the parenthesis $(91 \div 7 - 12)$ simplifies to 1.

The expression becomes:

$$[21 + 14 - 7 \times 1 + 42]$$

$$7 \times 1 = 7$$

The expression now is:

$$[21 + 14 - 7 + 42]$$

First, add 21 and 14:

$$21 + 14 = 35$$

Next, subtract 7 from 35:

$$35 - 7 = 28$$

Finally, add 42 to 28:

$$28 + 42 = 70$$

Thus, the value of the expression is 70.

Comparing this with the given options:

1. 80
2. 50
3. 70
4. 60

The calculated value matches option 3.