Single linkage is a measure of calculating dissimilarity between clusters by

a. considering the distance between the cluster centroids.
b. considering only the two most similar observations in the two clusters.
c. computing the average dissimilarity between every pair of observations between the two clusters.
d. considering only the two most dissimilar observations in the two clusters

Single linkage is a measure of calculating dissimilarity between clusters by

b. considering only the two most similar observations in the two clusters.

Explanation:

Single Linkage:

It is a type of hierarchical clustering  which is used to measure the dissimilarity between clusters by considering the only two observations that are closest in the clusters.

answer: a. the kind of population living in your neighbourhood.

the census explorer data tool by the us census bureau allows the user to zoom in on any neighbourhood within the united states and find the kind of people residing in that by race and age.  

the tool needs to needs to be used twice – once in order to find what type of people reside in a neighbourhood, and then to see the age groups of the people in the same neighbourhood.