Final answer:
Aleph-null (Option D) is the correct answer; it is a symbol not found in the Roman alphabet that is used to represent the cardinality of countably infinite sets in mathematics.
Step-by-step explanation:
The letter not found in the Roman alphabet that is used by mathematicians to identify cardinalities of infinite sets is Aleph-null (Option D). In set theory, Aleph-null (ℵ) is the smallest infinity and is used to denote the cardinality of the set of natural numbers (which is a countably infinite set).
The use of the Hebrew letter Aleph for this purpose was introduced by Georg Cantor, the founder of set theory, who also laid the groundwork for the concept of different sizes of infinity.
Different symbols from the Greek and other alphabets are commonly used in various branches of mathematics to represent various constants, functions, and set cardinalities. For instance, Δ (delta) represents a change in a quantity, while the Greek alphabet letters like alpha, beta, gamma, etc., have specialized meanings in contexts ranging from physics to astronomy, like the designation of stars in a constellation.