**Core Concept**
A frequency distribution is a representation, either in a graphical or tabular format, of the number of observations that fall within a given range of values. Non-symmetrical frequency distributions deviate from a normal or bell-shaped curve, which is a symmetrical distribution.

**Why the Correct Answer is Right**
A non-symmetrical frequency distribution is often seen in real-world data, where the left and right sides of the distribution are not mirror images of each other. This can occur due to various factors, such as the presence of outliers, skewness, or a non-normal population. In such cases, the distribution may be skewed to the left (negatively skewed) or right (positively skewed), resulting in a non-symmetrical shape.

**Why Each Wrong Option is Incorrect**
**Option A:**
This option is incorrect because a symmetrical frequency distribution, such as a normal distribution, is characterized by a bell-shaped curve, not a non-symmetrical shape.

**Option B:**
This option is incorrect because a bimodal distribution is a type of frequency distribution that has two distinct peaks, but it is still symmetrical.

**Option C:**
This option is incorrect because a skewed distribution is a type of non-symmetrical frequency distribution, but it is a more specific term that refers to the direction of skewness (left or right).

**Clinical Pearl / High-Yield Fact**
To remember the types of frequency distributions, recall the acronym "SKEW": Symmetrical (Normal), Skewed (Non-symmetrical), Bimodal (Two peaks).

**Correct Answer:** D. Skewed distribution.