ANSWER
Associative Property.
EXPLANATION
It was given that, variables S,T, and U are matrices.
The property of addition that was illustrated was,
S+(T+U)=(S+T)+U
On the left hand side, the last two matrices were added and the resulting matrix was added to the first matrix.
On the right hand side, the first two matrices were added, and the resulting matrix was added to the third matrix.
In both cases, the two results obtained were the same.
This property of matrix addition is called the associative property of addition.
The correct choice is the first option..
