The correct answer is B) Sheet bend. The sheet bend is the recommended knot for attaching two lines of different sizes or diameters together. This knot is secure and can accommodate the varying thicknesses of the lines being joined. In contrast, the other options are not as well-suited for this purpose - the thief knot (A) and granny knot (D) are less secure, while the square knot (C) is better suited for joining two lines of equal thickness. Using the proper knot, such as the sheet bend, is important for ensuring the safety and reliability of line connections on a vessel, which is a key requirement for Coast Guard captain's license examinations.