The structure which executes under certain conditions is decision structure.

Hence, the correct answer is option “C”.

Decision structure:

Decision structure executes a set of statements under certain conditions. It is also known as selection structure. The action is performed only when the condition exists and takes different action depending upon the state “True” or “False”.

Example for decision structure:

“if” statement:

 “if” statement is used to make a decision structure, which permits the program to hold more than one path of execution.

• The indented statement is executed only when the condition is “true”.
• If the given condition is “false”, it skips the indented statements.

Syntax for “if” statement:

if condition:

  indented_statement_block

Explanation for incorrect options:

Simplest form of control structure is the “sequence structure”. It is a group of statements that are executed in an order in which they appear.

Hence, option “A” is wrong.

Circumstantial structure does not execute the statements under certain conditions.

Hence, option “B” is wrong.

The Boolean expression evaluates the statement as either “True” or “False” with the help of “if” statement.

Hence, option “D” is wrong.