Answer
(a) Prefix code
(b) Not prefix code
(c) Prefix code
(d) Prefix code
Work Step by Step
(a) Prefix code
This code is prefix code. The encoding of $e$ as $00$, $a$ as $10$, $s$ as $01$ and $t$ as $11$.
(b) No prefix code
This code is not prefix code. The encoding of $e$ as $0$ not $1$, $a$ as $1$ not $0$.
(c) Prefix code
This code is prefix code. The encoding of $e$ as $11$, $a$ as $101$, $s$ as $011$ $t$ as $001$ and $n$ as $010$.
(d) Prefix code
This code is prefix code. The encoding of $e$ as $11$, $a$ as $010$, $s$ as $1011$ $t$ as $011$, $n$ as $1001$ and $i$ as $10101$.
