Answer
$\emptyset$; {a}; {b}; {c}; {d}; {a,b}; {a,c}; {a,d}; {b,c}; {b,d}; {c,d}; {a,b,c}; {a,b,d}; {b,c,d}; {a,b,c,d}.
Work Step by Step
The number of elements in a subset can be 0, 1, 2, 3, 4. Now we have to write all variation of elements: $\emptyset$; {a}; {b}; {c}; {d}; {a,b}; {a,c}; {a,d}; {b,c}; {b,d}; {c,d}; {a,b,c}; {a,b,d}; {b,c,d}; {a,b,c,d}.
