**Core Concept**
The anterior cruciate ligament (ACL) plays a crucial role in knee joint stability, specifically in controlling excessive anterior tibial translation and rotational movements. A deficient ACL compromises knee joint stability, affecting various activities that require these functions.

**Why the Correct Answer is Right**
A patient with an ACL-deficient knee joint will experience difficulty with activities that involve pivoting, cutting, or sudden changes of direction. This is because the ACL helps to stabilize the knee joint by limiting excessive anterior tibial translation and rotational movements. The ACL's deficiency will lead to increased laxity, making it challenging to perform activities that require quick changes in direction.

**Why Each Wrong Option is Incorrect**
* **Option A:** Activities like straight-line walking or running may not be significantly affected by an ACL-deficient knee joint, as these movements do not require significant pivoting or cutting.
* **Option B:** Jumping may be affected, but it is not the most specific or challenging activity for a patient with an ACL-deficient knee joint.
* **Option D:** Lifting heavy objects may not be directly affected by an ACL-deficient knee joint, as it does not require significant pivoting or cutting movements.

**Clinical Pearl / High-Yield Fact**
In patients with ACL-deficient knee joints, it's essential to consider the risk of meniscal tears and osteoarthritis, as the compromised knee joint stability can lead to increased stress on the menisci and articular cartilage.

**Correct Answer:** C.