The correct answer is C. Linearity.
A block code is a type of error-correcting code that encodes a message into a block of bits. The block code is designed to be able to correct errors that may occur during transmission.
There are many different types of block codes, but they all have some common properties. One of these properties is linearity. A linear code is a code in which the codeword $c$ corresponding to the message $m$ is a linear combination of the codewords corresponding to the message components $m_1, m_2, \ldots, m_n$. In other words, $c = a_1m_1 + a_2m_2 + \cdots + a_nm_n$ for some coefficients $a_1, a_2, \ldots, a_n$.
Linear codes are easier to analyze and decode than nonlinear codes. However, they are not as powerful as nonlinear codes. Nonlinear codes can correct more errors than linear codes.
The other options, A, B, and D, are all properties of block codes. A cyclic code is a code in which the codewords can be cyclically shifted to produce other codewords. A non-linear code is a code that is not linear. A systematic code is a code in which the message components are directly present in the codeword.