A syntax error occurs when the code does not conform to the rules of the programming language's syntax, leading to a failure in interpretation or compilation.
Let's examine each option:
A. Using one equal sign to compare two values - This is a common mistake in many programming languages (like using = instead of == in languages like C, Java, or Python for comparison), and while it may lead to logical errors, it is not a syntax error if the language accepts = as an assignment operator.
B. Concatenating strings instead of adding integers - This typically won't cause a syntax error but can lead to a type error or logical error depending on the context and the language being used.
C. Creating an infinite loop - This is not a syntax error either; it is a logical error. The code is valid syntactically but doesn't terminate as expected.
D. Using + to multiply two numbers - This will lead to a syntax error in many languages because the + operator is not valid for multiplication.
From the options provided, D (using + to multiply two numbers) is most likely to be classified as a syntax error in many programming languages.