Given,
Matrix A is symmetric, B is skew symmetric and C is skew symmetric,
So, AT=A,BT=−B,CT=−C
Now let M=A13B26−B26A13
Then, MT=(A13B26−B26A13)T
=(A13B26)T−(B26A13)T
=(BT)26(AT)13−(AT)13(BT)26
=B26A13−A13B26=−M
Hence, M is skew symmetric
Now let, N=A26C13−C13A26
Then, NT=(A26C13)T−(C13A26)T
=−(C)13(A)26+A26C13=N
Hence, N is symmetric.
So, only S2 is true.