Which of the following equations is written in standard form?

Standard Form is ax + by = c, where a, b, and c are not fractions and a is not negative.

So, you can go through each of your options to see which ones work with those rules. 

A. 2.5x + 3y = 12   No, this is not in Standard Form. 2.5 can be rewritten as 2, menaing A is a fraction, which you can't have.

B. -10x - 3y = 1   No, this is not in Standard Form. A is -10, but A can't be negative.

C. 2x + 3y = 12   Yes, this is in Standard Form. It follows all of the rules.

D. 5x + 5y = 10   Yes, this is in Standard Form. It follows all of the rules.

So, C and D are both written in Standard Form.