**Core Concept**
The non-enzymatic linkage of glucose with hemoglobin is a post-translational modification that occurs in diabetes mellitus. This process is known as glycation, which involves the covalent bonding of glucose molecules to the amino groups of hemoglobin.

**Why the Correct Answer is Right**
Glycation of hemoglobin occurs through a non-enzymatic reaction between glucose and the N-terminal valine of the beta-globin chain of hemoglobin. This reaction is facilitated by the presence of reducing sugars such as glucose, which can form a Schiff base with the amino group of hemoglobin. The Schiff base then undergoes an Amadori rearrangement to form a stable ketoamine linkage between glucose and hemoglobin. This process is accelerated in diabetes mellitus due to hyperglycemia.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specifically refer to the non-enzymatic linkage of glucose with hemoglobin. While glycosylation is a type of post-translational modification, it typically involves the enzymatic attachment of carbohydrate molecules to proteins.
**Option B:** This option is incorrect because it refers to a different type of post-translational modification that involves the addition of phosphate groups to proteins. Phosphorylation is an enzymatic process that is important in many cellular signaling pathways.
**Option C:** This option is incorrect because it refers to a type of protein modification that involves the addition of ubiquitin molecules to proteins. Ubiquitination is an enzymatic process that is important in protein degradation and cellular signaling.

**Clinical Pearl / High-Yield Fact**
The glycosylation of hemoglobin can be used as a marker for glycemic control in diabetes mellitus. The HbA1c test measures the percentage of hemoglobin that is glycosylated, providing a record of blood glucose levels over the past 2-3 months.

**Correct Answer:** A. Glycation