Explanation:

The attribute used to define cell contents to the left is the 'Align' attribute in HTML.

HTML Attributes:
HTML attributes provide additional information about HTML elements. They are used to modify the default behavior or appearance of an element.

The 'align' attribute:
The 'align' attribute is used to specify the horizontal alignment of content within an HTML element. It can be applied to various elements, including table cells (), table headings (), and table captions ().

Options:
a) VAlign - This attribute is used to specify the vertical alignment of content within an HTML element, not the horizontal alignment which is required to define cell contents to the left.

b) Align - This is the correct option. The 'align' attribute can be used with the element to define the horizontal alignment of cell contents. By setting the 'align' attribute to "left", the cell contents will be aligned to the left.

c) GAlign - There is no such attribute as 'GAlign' in HTML.

d) HAlign - The 'HAlign' attribute does not exist in HTML. It may be a typo or a mistaken option.

Example:

Let's consider the following HTML table structure:

```html

Cell 1

Cell 2

Cell 3

```

In the above example, the 'align' attribute is used with the element to align the content of each cell. The first cell's content will be aligned to the left, the second cell's content will be centered, and the third cell's content will be aligned to the right.

Conclusion:

In conclusion, the correct attribute to define cell contents to the left is the 'align' attribute in HTML. It is used in conjunction with the element and set to "left" to align the content to the left within the cell.