The correct answer is C) becket bend. The becket bend is a knot used to join two lines of different diameters. It is a secure and adjustable knot that can accommodate ropes or lines of varying sizes. The becket bend creates a strong connection between the two lines, making it a suitable choice when joining lines of dissimilar thickness. The other options are not the most appropriate choices for this scenario. The square knot is primarily used to join two lines of the same diameter, while the sheepshank and carrick bend are generally used for different purposes, such as forming a temporary loop or joining larger-diameter ropes, respectively.