In the C programming language, which of the following options shows a properly defined structure type declaration with a name and correct syntax?

Introduction / Context:
Structures in the C language allow you to group related variables into a single composite type. To use a structure, you must declare it with correct syntax so that the compiler knows both the tag name of the structure and the types of its members. This question asks which of the given options is a properly defined structure type declaration with a name and correct punctuation.

Given Data / Assumptions:

• We are using standard C syntax for defining structures.
• A typical structure type declaration begins with the keyword struct.
• A structure tag name is optional but recommended when you want to declare variables later using that type.
• Braces enclose the list of members, and the entire declaration ends with a semicolon.

Concept / Approach:
A proper structure declaration with a name uses the form struct tag_name { member declarations }; with a semicolon at the end. Each member declaration ends with its own semicolon. Without the final semicolon, the declaration is incomplete. An anonymous structure with no tag name is valid in some contexts but is less common when teaching basic syntax. Options that mix the tag and member declaration incorrectly, or that omit necessary punctuation, are not valid structure declarations.

Step-by-Step Solution:
Step 1: Examine option d: struct a_struct {int a;};. It starts with struct, gives a tag name a_struct, opens a brace, declares an int member a with a semicolon, closes the brace, and ends the entire declaration with a semicolon. This matches the standard pattern. Step 2: Option b, struct a_struct {int a;}, is missing the final semicolon after the closing brace, so the declaration is not complete in C. Step 3: Option c, struct a_struct int a;, places a member declaration outside of braces, which is invalid structure syntax. Step 4: Options a and e show an anonymous struct with an int member a but either miss the final semicolon or are incomplete as type declarations in this context. Step 5: Therefore, only option d provides a clean, properly defined structure type declaration with a name and full syntax.

Verification / Alternative check:
If you place option d into a small C program, for example above main, the compiler will accept it and allow you to declare variables using struct a_struct. For example, struct a_struct s; is valid. Trying to compile option c as written will produce syntax errors. Option b will also cause an error due to the missing semicolon. This real compiler behavior confirms that option d is the correct answer.

Why Other Options Are Wrong:
Option a lacks the final semicolon and has no tag name, which makes it incomplete for reuse as a named type. Option b is missing a semicolon at the end of the declaration. Option c mixes structure tag and variable syntax incorrectly. Option e shows an anonymous struct but adds a semicolon only once and does not provide a tag to reuse the type; in many exam contexts the focus is on the clearly named structure in option d.

Common Pitfalls:
Many students forget the semicolon after the closing brace in a structure declaration, which is a very common syntax error. Others confuse declaring a structure type with declaring a structure variable. Remember that the structure type declaration must wrap member declarations inside braces and end with a semicolon, and that the tag name allows later variable declarations using that type.

Final Answer:
The properly defined structure declaration is struct a_struct {int a;};, which appears in option d.