0.52 x 0.24 means "about a half times 0.24". Given the multiple choice answers, the correct answer is therefore 0.1224 (since this is the only one smaller than 0.24).
Proof:
0.50 × 0.24 = 0.12
0.01 × 0.24 = 0.0024
And therefore 0.51 × 0.24 = 0.12 + 0.0024 = 0.1224
