The associative property is a property that happnes with some operators in which the rearrengements of the perenthesis don't alter the result.
With this in mind, the alternatives that don't have any parenthesis can be excluded already.
Now, the same goes for the alternatives that have only parenthesis in one of the sides.
This excludes alternatives 1, 2 and 4.
The third alternative is:
[tex](2+3i)+5=2+(3i+5)[/tex]Notice that the only difference between the two sides are the position of the parenthesis, and this doesn't alter the result, so the third is the correct alternative.