Step 1: Recall the associative property of addition.
The associative property states that for any real numbers \( a \), \( b \), and \( c \):
\[
a + (b + c) = (a + b) + c.
\]
Step 2: Match the property to the options.
- (1) \( a + b = b + a \): This represents the commutative property of addition, not the associative property.
- (2) \( a + (b + c) = (a + b) + c \): This is the associative property of addition.
- (3) \( a(b + c) = ab + ac \): This represents the distributive property, not the associative property.
- (4) None of the above: Incorrect, as option (2) matches the associative property.
Final Answer: The correct option is \( \mathbf{(2)} \).