Final answer:
Among the options given, -7 (A), a single variable 'a' (B), and the number 1 (D) are monomials. Option C (x+y) is not a monomial because it consists of two separate terms.
Step-by-step explanation:
The question about monomials is asking to identify which of the given options are monomials. A monomial is an algebraic expression with only one term. It can be a constant number, a variable, or a product of constants and variables raised to non-negative integer powers.
In this case, the monomials among the options given are:
- A) -7 which is a constant number and therefore a monomial.
- B) a which is a single variable and also considered a monomial.
- C) x+y is not a monomial because it contains two terms separated by a plus sign.
- D) 1 which is the multiplicative identity and a monomial as well.
To sum up, options A, B, and D are monomials.