Key concept: Fat-soluble vs water-soluble vitamins

Vitamins are broadly classified into:

• Fat-soluble vitamins: $A, D, E, K$
• Water-soluble vitamins: B-complex vitamins and $C$

Fat-soluble vitamins are stored in the liver and fatty tissues for relatively long durations, whereas water-soluble vitamins are not stored significantly and are excreted more easily.

Identify each vitamin in the options

• Thiamine $=$ Vitamin $B_1$ r water-soluble
• Ascorbic acid $=$ Vitamin $C$ r water-soluble
• Vitamin A r fat-soluble
• Vitamin D r fat-soluble

Evaluate each option

A: Thiamine and Vitamin A

• Thiamine is water-soluble
• Vitamin A is fat-soluble
• Only one is stored for longer duration
• So, this pair is not correct

B: Ascorbic acid and Vitamin D

• Ascorbic acid is water-soluble
• Vitamin D is fat-soluble
• Only one is stored for longer duration
• So, this pair is not correct

C: Thiamine and Ascorbic acid

• Both are water-soluble
• These are not stored for long durations in the body
• So, this pair is not correct

D: Vitamin A and Vitamin D

• Both are fat-soluble
• Both are stored in the body for relatively longer duration
• So, this pair is correct

Final answer

The correct option is: $\boxed{D\text{: Vitamin A and Vitamin D}}$

Comparison with stored correct answer

Stored correct answer: $D$

My derived answer is also $D$, so they agree.