The correct answer is D. 1, 2 and 3.
Single-linkage, complete-linkage, and average-linkage are all metrics that can be used to find the dissimilarity between two clusters in hierarchical clustering.
Single-linkage is the simplest of the three metrics. It measures the distance between two clusters by taking the minimum distance between any two points in the clusters.
Complete-linkage is the most restrictive of the three metrics. It measures the distance between two clusters by taking the maximum distance between any two points in the clusters.
Average-linkage is a compromise between single-linkage and complete-linkage. It measures the distance between two clusters by taking the average distance between any two points in the clusters.
Each of these metrics has its own advantages and disadvantages. Single-linkage is very sensitive to outliers, while complete-linkage is very robust to outliers. Average-linkage is a good compromise between the two.
The choice of which metric to use depends on the specific data set and the desired results.