To rewrite **4/6 : 1/3** as a unit rate, we need to divide the numerator by the denominator. The unit rate is obtained when the denominator is equal to 1.
Let's calculate the unit rate:
**4/6 : 1/3** = **(4/6) ÷ (1/3)**
To divide fractions, we multiply the first fraction by the reciprocal of the second fraction:
**4/6 : 1/3** = **(4/6) × (3/1)**
Multiplying the numerators and denominators gives us:
**4/6 : 1/3** = **12/6**
Simplifying the fraction, we get:
**4/6 : 1/3** = **2**
Therefore, the unit rate for **4/6 : 1/3** is **2** ¹²³.
The correct answer is **B. 2:1**.