In general, for a new cultural trait to be adopted by a culture, it must offer some over an existing trait.

a. benefit
b. diversity
c. diffusion
d. drawback

For people to adopt something, it should generally be good in some way: this way people will want to adopt it.

One word for "good for someone" is "beneficial: so this is the correct answer.

Other answers either mean the opposite of beneficial (d) or just refer to the process of spreading (B and C).

If you will look at the sun through magnifying glass then the glass will defocus your eyes and so damage will be reduced.