Final answer:
The placeholder character often used in mathematics is X, which is used to represent an unknown value. The letter X is a consonant in the English alphabet, along with other letters that are not vowels. The vowels are always A, E, I, and O, with Y, W, and U sometimes acting as vowels.
Step-by-step explanation:
The alphabetic character that is always used as a placeholder character is B) X. In mathematics, X is commonly used to represent an unknown value or variable in equations and algebraic expressions. This convention helps in solving problems and formulating mathematical models. For instance, in the equation x + 2 = 5, the letter X is a placeholder for the unknown value we are trying to find.
In the following alphabet, cross off the letters that are always or sometimes vowels. a, e, i, o, u, y, w The nineteen letters not crossed off are always consonants. These consonants are <b, c, d, f, g, h, j, k, l, m, n, p, q, r, s, t, v, x, z>.
Four letters that are always vowels are <a, e, i, o>. These should be underlined to indicate their status as vowels in the English language. The letters W and Y can act as vowels or consonants depending on their use in words, while U is generally considered a vowel but can also act as a consonant in certain contexts like in the word 'university'.