**Explanation:**

Muscle fibers found in the wall of blood vessels are known as smooth muscle fibers or smooth muscle cells. These muscle fibers have specific features that distinguish them from other types of muscle fibers.

Let's analyze each option to determine which one is not a feature of muscle fibers found in the wall of blood vessels:

**a) Striated:**
Striated muscle fibers are characterized by the presence of alternating light and dark bands, giving them a striped or striated appearance under a microscope. This feature is not present in smooth muscle fibers. Smooth muscle fibers lack the organized arrangement of actin and myosin filaments that create the striations in skeletal and cardiac muscle fibers. Therefore, striated is not a feature of muscle fibers found in the wall of blood vessels.

**b) Uninucleate:**
Uninucleate means having a single nucleus. Smooth muscle fibers, including those found in the wall of blood vessels, are generally uninucleate. Unlike skeletal muscle fibers, which are multinucleate, smooth muscle fibers contain only one nucleus per cell.

**c) Involuntary:**
Smooth muscle fibers are involuntary in nature. This means that they contract and relax without conscious control. The contraction and relaxation of smooth muscle in blood vessel walls help regulate blood pressure and blood flow. Therefore, involuntary is a feature of muscle fibers found in the wall of blood vessels.

**d) Spindle-shaped:**
Smooth muscle fibers are characterized by their spindle-shaped appearance. They are elongated and tapered at both ends, resembling a spindle or cigar shape. This shape allows smooth muscle fibers to generate force and contract in a coordinated manner. Therefore, spindle-shaped is a feature of muscle fibers found in the wall of blood vessels.

In conclusion, the feature that is not associated with muscle fibers found in the wall of blood vessels is **striated** (option A). Smooth muscle fibers lack the striations present in skeletal and cardiac muscle fibers.

Answer a is correct as they need to contract and relax . so they are striated and flexible