The crispr system
a. synthesizes gene transfer agents during stationary phase.
b. recognizes foreign dna sequences that have previously entered the cell and directs the cas proteins to destroy them.
c. facilitates homologous recombination through a complex system of proteins and clustered repeats.
d. repairs dna and increases dna damage tolerance during times of stress.

The correct answer is b. recognizes foreign DNA sequences that have previously entered the cell and directs the Cas proteins to destroy them.

Explanation:

Clustered regularly interspaced short palindromic repeats (CRISPER) is the sequences of DNA found in the genome of prokaryotes and archaea. These sequences are added at a crisper locus in DNA by bacteria through capturing bacteriophage DNA during infection.

Then this sequence is used to detect a similar sequence of bacteriophage during subsequent infection and destroy them by Cas9 enzyme. Cas 9 enzyme uses a crisper sequence as a template to destroy similar phage DNA.  

So the CRISPER Cas system provides an adaptive defense mechanism to bacteria against foreign DNA coming from bacteriophage. Therefore the correct answer is b.

this protocol provides a detailed procedure for co-immunoprecipitation (co-ip) to identify protein–protein interactions.