A 30 N force is the minimum required to produce an acceleration of 2 m/s² on a 10 kg body. What is the frictional force on the body?

What the phrase “minimum force” tells you

If 30 N is the minimum force that still produces the given acceleration, then any smaller applied force would fail because friction would cancel too much of it. So friction must be taking away the extra force beyond what is needed for $ma$.

Use Newton’s second law to find the required net force

The net force needed for acceleration $a$ is:

$$F_{net}=ma=10\,\text{kg}\times 2\,\text{m/s}^2=20\,\text{N}$$

Relate applied force, friction, and net force

Taking the direction of motion as positive:

$$F_{applied}-F_f=F_{net}$$

Substitute values:

$$30-F_f=20 \quad\Rightarrow\quad F_f=10\,\text{N}$$

So the frictional force is 10 N, opposite the motion.