A One-to-One function has only one value of y for a given value of x. Neither x value is paired with more than 1 y value, nor a y value is paired with more than 1 x values.
From the given options, the option A represents a One-to-One function.
In option B, a same y value occurs at more than one values of x. Same can be seen in option C. Therefore, these two are not One-to-One functions.
So, the correct answer is option A.
