To determine which of the given expressions is a trinomial, we first need to understand what a trinomial is.

A trinomial is a polynomial with three terms. A polynomial is an algebraic expression that consists of variables, coefficients, and mathematical operations such as addition, subtraction, and multiplication.

Now let's evaluate each option to determine if it meets the criteria of a trinomial:

a) xyz: This expression consists of three variables multiplied together, but it does not have any addition or subtraction operations. Therefore, it is not a trinomial.

b) xy z: This expression consists of three variables, but there is a space between the variable 'y' and the variable 'z'. If the space is meant to indicate multiplication, then this expression would be written as 'xy * z' or 'xyz'. However, even with the correct formatting, this expression still does not have any addition or subtraction operations. Therefore, it is not a trinomial.

c) x y z: This expression consists of three variables, 'x', 'y', and 'z', and they are separated by spaces. Similar to the previous option, if the spaces are meant to indicate multiplication, then this expression would be written as 'x * y * z' or 'xyz'. This expression also does not have any addition or subtraction operations. Therefore, it is not a trinomial.

d) x yz: This expression consists of three terms, 'x', 'y', and 'z', and they are not separated by spaces. This expression also does not have any addition or subtraction operations. Therefore, it is not a trinomial.

So, the correct answer is option 'C' (x y z). It does not meet the criteria of a trinomial because it does not have any addition or subtraction operations.