I'm afraid you're not paying attention. This question is loaded with all the important words, and you don't recognize Newton's second law of motion. Didn't you notice the "force", "acceleration", and "mass" all over the place in there ?
Newton's 2nd law: Force = (mass) x (acceleration)
The answer is going to be 'mass', so let's divide each side of Newton's law by 'acceleration':
Mass = (force) / (acceleration)
Now, just write in the numbers that the question gave you:
force = 60 newtons acceleration = 4 m/s²
Here's the equation again: Mass = (force) / (acceleration)