Past exam of the communication Theory course of the University of Oxford / 2009 / 2 / a / i / Solution

In the terminology used here, a block code assigns a distinct word of the same length to every source symbol. For a binary code with $m$ symbols this common length must satisfy $2^{\ell }\ge m$. The equal-length property is what matters in this question; elsewhere the term can also refer more generally to coding blocks of source symbols.