In Python, integer literals can be written as a sequence of digits without any separators. Therefore, 1000000 is a valid representation of the integer one million. Since Python 3.6 (PEP 515), underscores can be used as visual separators for grouping digits in numeric literals to improve readability. The interpreter ignores these underscores. Thus, 1000000 is also a valid way to write the integer one million. Both methods result in a variable of type int with the value 1000000.

A. var = 1e6 creates a floating-point number (float), not an integer (int), due to the scientific 'e' notation.

C. var = 1.000.000 is a SyntaxError because periods are used for decimal points, and multiple decimal points in a number are not allowed.

E. var = 1,000,000 creates a tuple (1, 0, 0), as commas are used to separate elements in a tuple, not as digit groupers in a number.

1. Python Software Foundation. The Python Language Reference, Version 3.12. Section 2.4.4. Integer literals. The documentation states, "Underscores are permitted for grouping digits for enhanced readability. An underscore must be surrounded by digits or other underscores." This validates options B and D.

2. Python Software Foundation. PEP 515 -- Underscores in Numeric Literals. This official proposal, which was accepted and implemented, details the rationale and syntax for using underscores in numeric literals, directly supporting option B.

3. Python Software Foundation. The Python Language Reference, Version 3.12. Section 2.4.5. Floating-point literals. This section explains that the 'e' or 'E' notation is used to define floating-point numbers, which confirms why option A is incorrect for an int type.

4. Guttag, J.V. (2016). Introduction to Computation and Programming Using Python, With Application to Understanding Data. MIT Press. Chapter 2, Section 2.1 Objects, Expressions, and Numerical Types. This text clarifies that standard integer literals consist of a sequence of digits, and floating-point literals can use scientific notation.