## Core Concept
The question tests understanding of dental materials, specifically the properties and handling of zinc phosphate cement. Zinc phosphate cement is a type of dental cement used for luting crowns and bridges. Its properties can be influenced by the powder-liquid ratio during mixing.

## Why the Correct Answer is Right
An increased amount of powder in the zinc phosphate cement mixture will lead to a decrease in the setting time and an increase in the strength of the cement. This is because the powder (zinc oxide) reacts with the liquid (phosphoric acid) to form a hardened matrix. More powder increases the number of potential reaction sites, which can accelerate the setting reaction and enhance the mechanical properties of the set cement.

## Why Each Wrong Option is Incorrect
- **Option A:** Increased powder might affect the setting expansion, but saying it causes "increased setting expansion" is not accurate. The primary effect of more powder is not directly related to setting expansion.
- **Option B:** While more powder can make the mix thicker, saying it causes "less film thickness" is incorrect. A thicker mix (due to more powder) can actually lead to increased film thickness, not less.
- **Option C:** This seems to relate to solubility. Increased powder can decrease solubility because a more complete reaction might occur, reducing the amount of soluble components. However, this option does not directly address a commonly noted effect like increased strength or decreased setting time.

## Clinical Pearl / High-Yield Fact
A key point to remember is that altering the powder-liquid ratio of zinc phosphate cement affects its properties. Clinicians must carefully control this ratio to achieve desired handling properties and clinical performance. Too much powder can make the cement set too quickly, potentially compromising its seating or adjustment during clinical procedures.

## Correct Answer: D.