The correct answer is $\frac{{{\text{x}},{\text{y}}}}{{{\text{total number of transactions}}}}$.
Support is the percentage of transactions that contain both X and Y. It is calculated by dividing the number of transactions that contain both X and Y by the total number of transactions.
For example, if there are 100 transactions and 20 of them contain both X and Y, then the support of the association rule “If X and Y, then Z” is $\frac{20}{100} = 0.2$.
The other options are incorrect because they do not represent the support of the association rule.
Option A is the confidence of the association rule. It is calculated by dividing the number of transactions that contain both X and Y and Z by the number of transactions that contain X and Y.
Option B is the lift of the association rule. It is calculated by dividing the confidence of the association rule by the support of the association rule.
Option C is the specificity of the association rule. It is calculated by dividing the support of the association rule by the total number of transactions that contain Z.