Concept:
Formal gardens are based on order, balance, geometry and symmetry. They are generally planned in a fixed design.
Step 1: Check statement A.
Symmetry is the most important feature of formal gardens.
\[
A \text{ is correct}
\]
Step 2: Check statement B.
Formal gardens commonly use geometric shapes such as square, rectangular and circular designs.
\[
B \text{ is correct}
\]
Step 3: Check statement C.
Imitating nature and producing natural effects are features of informal gardens, not formal gardens.
\[
C \text{ is incorrect}
\]
Step 4: Check statement D.
In formal gardens, the land is adjusted or forced to fit the design plan.
\[
D \text{ is correct}
\]
Step 5: Check statement E.
Mixed natural planting is more common in informal gardens.
\[
E \text{ is incorrect}
\]
Therefore, the correct statements are:
\[
A,B,D
\]
\[
\therefore \text{Correct Answer is (C)}
\]