Why are immortal cell lines important for biological research?
a-they have two nuclei, allowing them to divide faster than normal cells.
b-they can replicate indefinitely and do not experience natural cell death.
c-they grow larger than normal cells so that they can divide into multiple cells.
d-they grow spontaneously rather than going through the process of mitosis.

B-They can replicate indefinitely and do not experience natural cell death.

Immortal cell lines is a population of cells, which would not proliferate indefinately in a cell culture plate unless a mutation does not have been occurred in them. The mutations occurring in these cells, can occur naturally or be intentionally induced for the purpose of experiment. These cells can replicate number of time and does not die as normal cell.  This property of immortal cell is useful for biological research because this helps to produce cell culture which have longer lifetime than the normal cells.

i think the answer is b.