Topic: Preposition,

Best Answer Explained in Below-:

The phrase "Unemployment is a great obstacle to our national development" uses the preposition "to" to indicate a relationship where unemployment hinders or obstructs the progress of national development. In this context, "to" is appropriate because it signifies that unemployment acts as a barrier or impediment preventing or delaying the achievement of a particular goal, which is national development.

Let's consider the other options for clarity:

- **A) at**: This preposition does not convey the correct relationship. It does not suggest that unemployment is an obstacle; rather, it implies a location or position.

- **C) of**: While this preposition might suggest a characteristic, it would incorrectly imply that unemployment is a defining feature of national development, which distorts the intended meaning.

- **D) in**: This option could imply a context or area where unemployment exists but fails to communicate how unemployment impacts developmental goals.

In summary, "to" effectively communicates that unemployment is a significant barrier affecting the progress towards national development, making it the most suitable choice for this sentence.