**Core Concept**
The synthesis of immunoglobulins (antibodies) in both membrane-bound and secretory forms is a crucial process in the immune response. This process involves the coordinated action of multiple enzymes and cellular components to produce a functional antibody that can either remain attached to the cell surface or be released into the circulation.

**Why the Correct Answer is Right**
The correct answer is related to the process of class-switch recombination, which determines whether an immunoglobulin is synthesized in the membrane-bound or secretory form. In the case of membrane-bound immunoglobulins, the enzyme terminal deoxynucleotidyl transferase (TdT) plays a key role in the process. However, for secretory immunoglobulins, the enzyme terminal deoxynucleotidyl transferase (TdT) is not involved, and the class of the immunoglobulin is determined by the somatic recombination of variable, diversity, and joining (V, D, and J) gene segments. The correct answer is **Aicda**, which encodes the enzyme activation-induced cytidine deaminase (AID). AID is essential for the process of class-switch recombination, which determines the class of immunoglobulin produced.

**Why Each Wrong Option is Incorrect**
* **Option B:** This is incorrect because terminal deoxynucleotidyl transferase (TdT) is not involved in the synthesis of secretory immunoglobulins.
* **Option C:** This is incorrect because the enzyme terminal deoxynucleotidyl transferase (TdT) is not a key player in the process of class-switch recombination.
* **Option D:** This is incorrect because the enzyme terminal deoxynucleotidyl transferase (TdT) is not directly involved in the synthesis of secretory immunoglobulins.

**Clinical Pearl / High-Yield Fact**
A key aspect of immunoglobulin synthesis is the process of somatic hypermutation, which allows for the generation of high-affinity antibodies. This process is also mediated by the enzyme activation-induced cytidine deaminase (AID).

**Correct Answer:** A. Aicda.