In algebra, unknown quantities are typically represented by lower case letters. Option B, "Lower case letters," is the correct answer.

Explanation:
Algebra is a branch of mathematics that deals with the manipulation and representation of symbols to solve equations and analyze mathematical relationships. In algebraic expressions and equations, letters are commonly used to represent unknown quantities or variables.

Here's a detailed explanation of each option:

a) Upper case letters:
While upper case letters can be used for variables in certain contexts, such as representing matrices or sets, they are not typically used to represent unknown quantities in algebraic expressions. For example, uppercase letters like A, B, C are often used to represent constants or known values, rather than unknowns.

b) Lower case letters:
Lower case letters, such as x, y, and z, are commonly used to represent unknown quantities in algebra. When solving equations or working with algebraic expressions, we often assign lower case letters as variables to represent unknown values. For example, if we are trying to find the value of a missing side length in a geometric shape, we might use the variable "x" to represent that unknown length.

c) Depends on the problem:
While it is true that the choice of variables can depend on the specific problem being solved, lower case letters are generally the most commonly used convention to represent unknown quantities in algebra. The specific variable chosen may vary based on the context or nature of the problem, but lower case letters are a standard choice for representing unknowns.

d) Only letters A, B,..... K:
Using only letters from A to K to represent unknown quantities in algebra would be limiting. Algebra allows for a wide range of variables, including all letters of the alphabet, both upper and lower case. Restricting the variables to the letters A to K would not encompass the full range of possibilities.

In conclusion, lower case letters are commonly used to represent unknown quantities in algebra. While other options may have specific uses or circumstances, lower case letters are the most widely used convention for representing unknowns in algebraic expressions and equations.