Consider the given expression:
a+bi+c+di = a+c+bi+di
We have to identify the property of addition in the above expression.
Since, we can observe here that (bi+c) is expressed as (c+bi).
Commutative property of addition states that the numbers can be added in any of the order. Mathematically, it states that a+b = b+a.
Here, (bi+c) is replaced by (c+bi), whether 'b' is added to 'ci' or 'ci' is added to 'b', the result is always same.
Therefore, commutative property of addition is the correct answer.