Here in this question s= (a + b*)* when we open it it will become { ε, a, b, a^2, a^3, .., b^2, b^3,.. ab, abb... }* And we know that to make it sigma* we need only (a+ b)* this deadly combo to generate all this strings so ultimately it will become sigma * .
And given T is automatically in this form (a + b)* so it is also a sigma* that's why both are equal .