He spent 1/4 on Monday, so he had 3/4 left.
He then spent 2/3 of that, to find how much he spent you would multiply 3/4 by 2/3:
3/4 x 2/3 = 6/12 = 1/2
On Tuesday he spent 1/2 of his allowance.
So on Monday and Tuesday he spent 1/4 + 1/2 = 3/4 total.
He would have 1/4 of his allowance left.
The answer is C.