The correct answers are: The thief left few clues behind, making the case almost impossible to solve. We have a little evidence against the suspect, which gives us a solid lead! Fortunately, a few witnesses came forward to share what they saw.

Few and a few are used with plural countable nouns (clues, witnesses, suspects). Without the "a", few emphasizes a negative lack of something. With the "a", a few means a positive "some."

Little and a little are used with uncountable nouns (evidence, dust).

Incorrect options explained:

"Suspects" is countable, so it requires "few," not "little."

"Dust" is uncountable, so it requires "little," not "few."