Which of the following represents a phonogram?

A phonogram is a group of letters that represent a single sound or a sound combination in a specific context. In the context of phonics, a phonogram typically consists of a vowel followed by one or more consonants, which allows learners to decode the spelling-sound correspondence in words.

The choice 'ough' serves as a phonogram because it can represent various sounds depending on its usage in different words, making it a versatile phonogram in English. For instance, it can be found in words like "though," "through," and "cough," where it takes on different pronunciations.

In contrast, while the other choices (ch, st, and bl) represent consonant blends or digraphs rather than phonograms, they do not encapsulate the concept of a phonogram that combines both vowels and consonants to create variable sound correspondences. Instead, each of those choices represents specific sounds without the complexity and variability that 'ough' embodies, which highlights the unique qualities that make it the correct choice in relation to the definition of a phonogram.