ANS:A - avoid buckling of tubes.

The purpose of a floating head in a heat exchanger is to facilitate its lengthening, if needed. In a floating head heat exchanger, one of the tube sheets (usually the front one) is fixed in place, while the other tube sheet (the floating head) is free to move back and forth within the shell. This design allows for thermal expansion and contraction of the tubes without causing excessive stress or damage. Additionally, the floating head design allows for easier access to the tubes for maintenance and cleaning purposes. However, it's important to note that while a floating head can accommodate thermal expansion, it doesn't directly prevent buckling of tubes, provide support for tubes, or decrease pressure drop. These are typically addressed through other design features or considerations.