The sweet smelling compound [D] is:

The correct option is Ester.

Let's understand why this is the correct answer step-by-step:

1. Identification of Sweet-Smelling Compounds:
In organic chemistry, esters are well-known for having characteristic, pleasant, and sweet fruity odors. Many natural fruits and flowers owe their distinctive fragrances to the presence of specific ester molecules. For example, ethyl butyrate smells like pineapple, and pentyl acetate smells like banana.

2. Comparison with Other Functional Groups:
Let's look at the other functional groups listed in the options:
- Acid anhydrides typically have sharp, irritating, or pungent odors due to their hydrolysis to carboxylic acids in air.
- Acyl halides (such as acetyl chloride) react vigorously with moisture in the air to release hydrogen halide gases (like HCl), resulting in an extremely sharp, suffocating, and pungent smell.
- Aldehydes can have a range of odors; while some higher aldehydes have floral or fruity smells, lower aldehydes (like formaldehyde or acetaldehyde) have very pungent and irritating odors. They are not generally characterized as the classic "sweet-smelling compounds" in introductory chemistry contexts.
- Esters, on the other hand, are universally introduced in chemistry as the class of organic compounds characterized by a pleasant, sweet, or fruity smell, often produced via the esterification reaction between a carboxylic acid and an alcohol.

Therefore, the sweet-smelling compound [D] is an Ester.