**Core Concept**
Glycoproteins are a class of proteins that are covalently linked to carbohydrate or sugar molecules. This glycosidic bond is formed between the carbohydrate moiety and the protein backbone, often through an amino acid residue such as asparagine or arginine. The carbohydrate portion of a glycoprotein typically consists of a core structure with attached sugar residues.

**Why the Correct Answer is Right**
The correct answer, option D (ad), accurately describes glycoproteins. Option A, "Protein linked with glycosidic bond," is a true statement as glycoproteins are indeed linked to carbohydrate molecules through a glycosidic bond. Option D also correctly states that glycoproteins "Participate in cell surface recognition," which is a key function of these molecules. They play a crucial role in cell signaling, cell adhesion, and immune responses by interacting with other cell surface molecules.

**Why Each Wrong Option is Incorrect**
**Option B:** "Core protein" is not entirely accurate as the core protein is just a part of the glycoprotein, and it does not encompass the entire molecule. It is the protein backbone to which the carbohydrate moiety is attached.
**Option C:** "Sugar residues are long in carbohydrate portion of glycoprotein" is incorrect because the length of sugar residues can vary greatly among different glycoproteins. Some glycoproteins have short sugar chains, while others have long, branched chains.

**Clinical Pearl / High-Yield Fact**
Glycoproteins are essential for cell-to-cell interactions and immune responses. The carbohydrate portion of glycoproteins can be altered in certain diseases, leading to changes in cell surface recognition and function. For example, changes in glycoprotein expression have been implicated in cancer progression and metastasis.

**✓ Correct Answer: D. ad. Glycoproteins are linked to carbohydrate molecules through a glycosidic bond and participate in cell surface recognition.**