Choice Bhoice D is the correct answer.

1. Start with the equation: $7x + 21 = 42$
2. Subtract 21 from both sides: $7x = 42 - 21$
3. Simplify: $7x = 21$
4. Divide both sides by 7: $x = \frac{21}{7}$
5. Simplify: $x = 3$
6. Verify: $7(3) + 21 = 21 + 21 = 42$ ✓

Choice Bhoice C is incorrect because this results from adding 21 to 42: $63$, then $63/7 = 9$. Wrong inverse operation.

Choice Bhoice C is incorrect because this is the value of $7x$. You need to divide by 7.

Choice Bhoice D is incorrect because this results from multiplying 21 by 7: $147$.