Final answer:
The open sentence in the given options is 3(x) = 12.
Step-by-step explanation:
The open sentences in the given options are:
- A. 3(x) = 12: This is an open sentence because it includes a variable, 'x', which can take different values.
- B. 12 = 4(2) + 3: This is not an open sentence because it does not contain any variables.
- C. 7 + 5 = 2: This is not an open sentence because it is a statement of equality that is either true or false, and does not contain any variables.
- D. 6(2) = 12: This is not an open sentence because it does not contain any variables.
Therefore, option A (3(x) = 12) is the correct answer.