Introduction:
Digital Signal Processors (DSPs) are specialized microprocessors designed to efficiently process digital signals in real-time applications such as audio and video processing, telecommunications, and control systems. The architecture of a DSP plays a crucial role in determining its performance, including execution time.

Explanation:
The architecture of a digital signal processor can greatly impact its execution time. Among the given options, the Harvard architecture is known for reducing execution time due to its unique characteristics. Let's understand why:

Harvard Architecture:
The Harvard architecture is a computer architecture that separates the instruction memory and data memory into two separate units. In this architecture, instructions and data are fetched from separate memory units simultaneously, allowing for parallel processing. This separation of memory units enables the processor to fetch multiple instructions and data in parallel, which reduces the overall execution time.

Advantages of Harvard Architecture:
1. Parallel Instruction and Data Fetching: The separation of instruction and data memory units allows for simultaneous fetching of instructions and data, enabling parallel processing. This reduces the time required to fetch and execute instructions, resulting in faster execution.

2. Reduced Instruction Fetch Time: With separate instruction memory, the processor can fetch instructions without waiting for data transfers. This reduces the instruction fetch time, leading to faster execution.

3. No Instruction/Data Bottleneck: By having separate memory units, the Harvard architecture eliminates the bottleneck that can occur when instructions and data share a common memory. This ensures a smooth flow of instructions and data, improving overall performance.

4. Increased Bandwidth: The separation of memory units allows for a wider memory bus, increasing the bandwidth available for fetching instructions and data. This increased bandwidth further speeds up the execution time.

5. Efficient Instruction Pipeline: The Harvard architecture enables efficient instruction pipelining, where multiple instructions can be processed simultaneously. This increases the instruction throughput and reduces the execution time.

Conclusion:
In summary, the Harvard architecture in a digital signal processor reduces the execution time by enabling parallel instruction and data fetching, reducing instruction fetch time, eliminating instruction/data bottlenecks, increasing bandwidth, and supporting efficient instruction pipelining. These advantages make the Harvard architecture a preferred choice for real-time signal processing applications.