The first character of a(n)__ must be a letter or an undeerscore(_).

a.class
b.identifier
c.module
d.program

please explain

To find the answer, you need to understand what each option represents.

a. `class`: In programming, a class is a blueprint or template for creating objects, which are instances of the class. In this case, the first character of a class can be any letter or underscore.

b. `identifier`: An identifier is a name given to a program element, such as a variable, function, or class. The first character of an identifier can also be any letter or underscore.

c. `module`: A module is a file containing Python definitions and statements. When using modules, the first character of the module name can be any letter or underscore.

d. `program`: A program is a set of instructions that a computer can execute. In this context, the first character of a program can be any letter or underscore.

Considering the given options, the correct answer is `b. identifier` because, according to the statement, the first character of an identifier can be a letter or an underscore.

In summary, the first character of an identifier can be a letter or an underscore in programming.