Fill in the blank with an appropriate option.

In humans, lack of sleep leads to ______ memory and reduced cognitive abilities.

impaired

The correct option to fill in the blank is impaired.

The word "impaired" means "reduced in strength, quality, or ability". It is the most appropriate word to use in this context, as lack of sleep can lead to a reduction in memory and cognitive abilities.

The other options are not as appropriate:

• Attended means "to be present at something". It would not be accurate to use "attended" in this context, as lack of sleep does not mean that you are not present.
• Thankless means "not giving or receiving thanks". It would not be accurate to use "thankless" in this context, as lack of sleep is not related to thanks.
• Witnessed means "to see or experience something". It would not be accurate to use "witnessed" in this context, as lack of sleep does not mean that you see or experience something.