Final answer:
In the context of naming variables in programming, both 'country' and '_country' are typically valid character formats, whereas '$ctry' and '$country' may only be valid in certain languages like PHP.
Step-by-step explanation:
The question is asking for a valid name for a character format which refers to the naming rules for variables in programming.
A variable name in most programming languages can start with a letter or an underscore, and may contain letters, digits, and underscores, but cannot begin with a digit or contain certain special characters or spaces. Therefore, options a (country) and d (_country) are potentially valid character format names depending on the specific programming language's rules. Option b ($ctry) and option c ($country) may be valid in some languages, such as PHP, where variable names can start with a dollar sign, however, in many other programming languages this is not allowed.