Explanation:
The correct option is 'a'. The Tip-Apex Distance (TAD) is the sum of the distance from the tip of the lag screw to the apex of the femoral head on both AP and lateral radiographs, after correcting for magnification. Baumgaertner et al. demonstrated that a TAD of less than 25 mm significantly minimizes the risk of the screw cutting out of the osteoporotic femoral head.

Reference: Baumgaertner MR, Curtin SL, Lindskog DM, Keggi JM. The value of the tip-apex distance in predicting failure of fixation of peritrochanteric fractures of the hip. J Bone Joint Surg Am. 1995;77(7):1058-1064.