The correct answer is C. 4950.
The Apriori algorithm is a frequent itemset mining algorithm that finds all frequent item sets in a database. It works by first finding all the 1-itemsets, then finding all the 2-itemsets that are frequent, and so on.
The number of candidate 2-itemsets is equal to the number of combinations of 2 items from the set of 100 items, which is $100 \choose 2 = 4950$.
Option A is incorrect because it is the number of 1-itemsets. Option B is incorrect because it is the number of 2-itemsets that are frequent. Option D is incorrect because it is the total number of possible 2-itemsets.