a) Without compression, they might become sparse
b) Accessing individual bits is expensive
c) Compressing bit array to byte/word array, the machine also has to support byte/word addressing
d) Storing and Manipulating in the register set for long periods of time

Answer: d
Explanation: Bit arrays allow small arrays of bits to be stored and manipulated in the register set for long periods of time with no memory accesses because of their ability to exploit bit-level parallelism, limit memory access, and maximally use the data cache, they often outperform many other data structures on practical data sets. This is an advantage of bit array. The rest are all disadvantages of bit array.