D. of doing
Correction: "I have never dived in a twenty-metre-deep lake before, so I'm a bit afraid to do it."
("Afraid of doing" should be "afraid to do.")

A. have
Correction: "You have little to boast about, don't you?"
("Haven't you?" should be "don't you?" in this context.)

C. worth
Correction: "Not surprisingly, poverty is a problem worth concern in every country."
("Worth of concern" should be "worth concern.")

A. is an
Correction: "This recipe, which is an old family secret, is an especially important part of our holiday celebrations."
("This recipe, is an" should be "This recipe, which is an.")