12ad^3 and which terms are like terms?

15a^3d

12d^3

3ad^3

10ad

Like terms are terms with the exact same variables, and the exact same degree of each variable ... simply put, the exponents of each variable have to be exactly the same.

The given term 12 ad³ has variables a and d. a has a degree of 1 and d has a degree of 3.

The term 3ad³ has the same variables and the same degree, and is therefore the like term.