**Core Concept**
Immunosuppressive drugs work by inhibiting the immune system's response to a transplanted organ, thereby preventing rejection. However, their effectiveness varies depending on the type of rejection mechanism involved. Acute cellular rejection is mediated by T cells, whereas acute humoral rejection involves the activation of B cells and the production of antibodies.

**Why the Correct Answer is Right**
Acute humoral rejection, also known as antibody-mediated rejection, is a type of rejection that occurs when the immune system produces antibodies against the graft. This process is resistant to conventional immunosuppressive drugs, which primarily target T-cell activation and proliferation. The production of antibodies by B cells is a complex process involving multiple cell types and cytokines, making it challenging for immunosuppressive drugs to effectively suppress. In contrast, acute cellular rejection is more responsive to immunosuppression, as it is primarily driven by T-cell activation.

**Why Each Wrong Option is Incorrect**
**Option A:** Acute cellular rejection is actually more responsive to immunosuppressive drugs than acute humoral rejection. Immunosuppressive drugs can effectively suppress T-cell activation and proliferation, making acute cellular rejection a more manageable condition.

**Option C:** Chronic rejection is a type of rejection that occurs over a longer period and is characterized by a gradual decline in graft function. While it is more difficult to manage than acute rejection, it is not necessarily more resistant to immunosuppressive drugs than acute humoral rejection.

**Option D:** None of the above is incorrect because there is a specific type of rejection that is less responsive to immunosuppressive drugs, which is acute humoral rejection.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that acute humoral rejection requires a different treatment approach than acute cellular rejection. This includes the use of antibody-depleting agents, such as plasmapheresis and intravenous immunoglobulin, in addition to conventional immunosuppressive drugs.

**✓ Correct Answer: B. Acute humoral rejection**