The correct answer to the question is option 'A', which represents Calcium (Ca).

The middle lamella is a layer that is found between adjacent plant cells. It is composed of a complex mixture of polysaccharides, such as pectins, and also contains various ions. Among these ions, calcium (Ca2+) is the main element that occurs in the middle lamella. Here is a detailed explanation of why calcium is the primary element in the middle lamella:

1. Middle Lamella Function:
The middle lamella serves as a cementing layer between plant cells, holding them together to form tissues. It plays a crucial role in cell adhesion and provides mechanical strength to the plant tissues.

2. Composition of Middle Lamella:
The middle lamella is primarily composed of pectins, which are a group of complex polysaccharides. Pectins are present as calcium pectate, where calcium ions cross-link the pectin molecules together.

3. Role of Calcium:
Calcium ions (Ca2+) play a vital role in the formation of calcium pectate. They form bridges between adjacent pectin molecules, creating a network of cross-links. This cross-linking strengthens the middle lamella and enhances cell adhesion.

4. Calcium's Presence:
Calcium is abundant in plant cells and is actively transported into the middle lamella. Once inside the middle lamella, calcium ions bind to the carboxyl groups of pectin molecules, forming calcium pectate complexes. These complexes contribute to the stability and rigidity of the middle lamella.

5. Other Elements:
While calcium is the main element in the middle lamella, other ions such as magnesium (Mg2+), sodium (Na+), and potassium (K+) may also be present. However, their concentrations are comparatively lower than calcium.

In conclusion, calcium is the primary element found in the middle lamella. It forms calcium pectate complexes with pectin molecules, contributing to the adhesion and mechanical strength of plant cells and tissues. The presence of calcium ions in the middle lamella is essential for maintaining the structural integrity of plant tissues.