Concept:
Most fruits contain a high amount of water, carbohydrates, vitamins and minerals, but very low fat content. However, some fruits are exceptional because they contain a comparatively high amount of fat.
Step 1: Check mango.
Mango is rich in carbohydrates, carotenoids and vitamins, but it is not known for high fat content.
\[
\text{Mango} \neq \text{high fat fruit}
\]
Step 2: Check grape and water apple.
Grape and water apple are juicy fruits. They mainly contain water, sugars and minerals, but they do not contain very high fat.
\[
\text{Grape and water apple} \rightarrow \text{low fat fruits}
\]
Step 3: Check avocado.
Avocado is well known for having a high level of fat, especially healthy unsaturated fats.
\[
\text{Avocado} \rightarrow \text{high fat fruit}
\]
Therefore, the fruit crop with very high level of fat is avocado.
\[
\therefore \text{Correct Answer is (C)}
\]