Boolean attributes are attributes in HTML that can have only two values: true or false. They do not require a value to be specified and are used to indicate the presence or absence of a particular attribute.

In the given options, the attribute "formtarget" is not a Boolean attribute. Let's understand this in detail:

HTML Boolean Attributes:
Boolean attributes can be specified in HTML without any value. If a boolean attribute is present, its value is considered to be true. On the other hand, if a boolean attribute is absent, its value is considered to be false.

Examples of Boolean attributes in HTML are:
1. Multiple: This attribute is used in the element to allow multiple options to be selected.
2. Novalidate: This attribute is used in the

element to indicate that the form should not be validated when submitted.
3. Formvalidated: This attribute is not a valid HTML attribute.

Explanation of the options:

a) Multiple: This attribute is a Boolean attribute that allows multiple options to be selected in a

element.

b) Novalidate: This attribute is a Boolean attribute that indicates that the form should not be validated when submitted.

c) Formvalidated: This attribute is not a valid HTML attribute. It does not exist in the HTML specification.

d) Formtarget: This attribute specifies where to display the response received after submitting a form. It is not a Boolean attribute as it requires a value to be specified (e.g., "_blank", "_self", "_parent", "_top", or a frame name).

Conclusion:
Out of the given options, the attribute "formtarget" is not a Boolean attribute. It requires a value to be specified and is used to determine where the form response should be displayed.