Select the correct statements.
I. Oxide ores of heavy metals are concentrated by hydraulic washing.
II. Froth floatation process is used for the concentration of magnetic ores.
III. Oxides of highly reactive metals can be reduced to metals by the action of heat alone.
IV. Carbonate ores are decomposed to form metal oxides by heating in the absence of air.

Options

A. I and IV only
B. II and III only
C. I, III and IV only
D. I, II, III and IV

Explanation

To solve the question, we need to evaluate each statement one by one to determine if they are correct or incorrect.

**Step 1: Analyze Statement I**
- Statement: "Oxide ores of heavy metals are concentrated by hydraulic washing."
- Explanation: Hydraulic washing is a method used to separate heavier ores from lighter impurities based on density. Since heavy metal ores are denser than the impurities, this statement is correct.

**Step 2: Analyze Statement II**
- Statement: "Froth flotation process is used for the concentration of magnetic ores."
- Explanation: The froth flotation process is primarily used for the concentration of sulfide ores, not magnetic ores. Magnetic ores are concentrated using electromagnetic separation. Therefore, this statement is incorrect.

**Step 3: Analyze Statement III**
- Statement: "Oxides of highly reactive metals can be reduced to metals by the action of heat alone."
- Explanation: Highly reactive metals typically cannot be reduced to their elemental form by heat alone; they require more energy, often through electrolysis. Thus, this statement is incorrect.

**Step 4: Analyze Statement IV**
- Statement: "Carbonate ores are decomposed to form metal oxides by heating in the absence of air."
- Explanation: This process is known as calcination, where carbonate ores are heated to produce metal oxides without air. This statement is correct.

**Final Evaluation:**
- Correct Statements: I and IV
- Incorrect Statements: II and III

Thus, the correct statements are I and IV.

### Summary of the Solution:
- Statement I: Correct
- Statement II: Incorrect
- Statement III: Incorrect
- Statement IV: Correct