Answer
$
a-T(1)=\left \{ 1 \right \}\\
b-T(15)=\left \{ 1,3,5,15 \right \}\\
c-T(17)=\left \{ 1,17 \right \}\\
d-T(5)=\left \{ 1,5 \right \}\\
e-T(18)=\left \{ 1,2,3,6,9,18 \right \}\\
f-T(21)=\left \{ 1,3,7,21 \right \}\\
$
Work Step by Step
$T:Z^+\rightarrow D \\
T(n)=\,the\,set\,of\,positive\,divisors\,of\,\,n.\\
divisors\,of\,\,1 :1\\
T(1)=\left \{ 1 \right \}\\
divisors\,of\,\,15:1,3,5,15 \\
T(15)=\left \{ 1,3,5,15 \right \}\\
divisors\,of\,\,17:1,17\\
T(17)=\left \{ 1,17 \right \}\\
divisors\,of\,\,5:1,5\\
T(5)=\left \{ 1,5 \right \}\\
divisors\,of\,\,18:1,2,3,6,9,18\\
T(18)=\left \{ 1,2,3,6,9,18 \right \}\\
divisors\,of\,\,21:1,3,7,21\\
T(21)=\left \{ 1,3,7,21 \right \}\\
$
