In data communication and text processing, what are digrams and trigrams commonly used to refer to?

Introduction / Context:
The terms digram and trigram arise in the study of coding, cryptography, natural language processing, and data compression. They refer to patterns in sequences of symbols such as characters or bits. Although these concepts are not restricted to networking, they affect how data is modelled and compressed before transmission. This question checks whether you know what digrams and trigrams are in the context of symbol sequences.

Given Data / Assumptions:

- We are considering text or data represented as sequences of symbols such as characters or bytes.- The question asks about the meaning of digrams and trigrams.- Options reference characters, network topologies, parity bits, IP addresses, and user groups.

Concept / Approach:
A digram is a sequence of two symbols, and a trigram is a sequence of three symbols. In natural language processing, digrams and trigrams of letters or words are used to model language statistics. In cryptanalysis, analysts study the frequency of digrams and trigrams to break substitution ciphers. In data compression, probabilities of symbol pairs and triples are used to design efficient codes. These constructs are therefore key tools in frequency analysis and statistical modelling of symbol sequences, not in physical network topology or address planning.

Step-by-Step Solution:
Step 1: Break down the words. The prefix di indicates two, and tri indicates three, suggesting pairs and triples.Step 2: Recognise that in text processing, simple models often start with single character frequency, then extend to digram (pairs) and trigram (triples) frequency.Step 3: Option A states that digrams and trigrams are sequences of two and three characters or symbols used in frequency analysis and coding of text or data streams, which aligns with common usage.Step 4: Option B describes two and three dimensional network topologies, which is unrelated to the standard use of these terms.Step 5: Option C treats digrams and trigrams as parity bits, confusing them with error detection codes such as Hamming codes.Step 6: Option D suggests they are pairs and triplets of IP addresses, which is not standard terminology.Step 7: Option E relates to groups of users sharing credentials, which is a security concept and not connected to digrams or trigrams.Step 8: Confirm that option A matches textbook definitions.

Verification / Alternative check:
Cryptography and language modelling literature frequently defines digrams as two symbol sequences and trigrams as three symbol sequences. Examples include pairs of letters like TH in English and triples like THE, which have high frequency and are useful in predicting text. Compression algorithms and Markov models also leverage digram and trigram statistics. Nowhere in networking or security standards are these words used to describe topologies, address groups, or parity bits, reinforcing that option A is the correct interpretation.

Why Other Options Are Wrong:
Network topologies are described using terms like bus, ring, star, mesh, and tree, not digrams or trigrams. Parity bits and error correcting codes may use parity check matrices and syndrome calculations, but they do not use this terminology. Grouping IP addresses for redundancy is referred to as clustering or virtual router redundancy, again without using digram or trigram. Sharing login credentials is discouraged from a security standpoint and is not named in this way either.

Common Pitfalls:
Because the question appears under a networking category, some learners assume that digram and trigram must be network specific concepts. It is important to realise that exams often test cross cutting ideas from information theory and cryptography that support data transmission. When in doubt, remember the root meanings of prefixes like di and tri and consider in which fields those terms are actually used.

Final Answer:
Digrams and trigrams are sequences of two and three characters or symbols used in frequency analysis and coding of text or data streams, as in option A.