**Core Concept**
Porcelain laminate veneers and Maryland bridges are types of conservative dental restorations used to replace or repair damaged teeth. The retention of these restorations is crucial for their long-term success. The retention of these restorations is achieved through mechanical means, such as micro-mechanical retention and mechanical retention.

**Why the Correct Answer is Right**
The correct answer is mechanical retention. Mechanical retention in porcelain laminate veneers and Maryland bridges is achieved through the use of retention grooves, boxes, or channels in the tooth preparation. These mechanical features provide a mechanical lock that helps to prevent the restoration from coming loose over time. The mechanical retention is often supplemented by micro-mechanical retention, which is achieved through the use of etching agents that create microscopic irregularities on the surface of the tooth and restoration.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because chemical retention is not a primary means of retention for porcelain laminate veneers and Maryland bridges. While chemical bonding agents may be used to bond the restoration to the tooth, this is not the primary mechanism of retention.

**Option B:** This option is incorrect because micro-mechanical retention, while important, is not the primary means of retention for porcelain laminate veneers and Maryland bridges. Micro-mechanical retention is often used in conjunction with mechanical retention to provide additional stability.

**Option D:** This option is incorrect because adhesive retention is not a primary means of retention for porcelain laminate veneers and Maryland bridges. While adhesive bonding agents may be used to bond the restoration to the tooth, this is not the primary mechanism of retention.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the success of porcelain laminate veneers and Maryland bridges depends on the proper preparation of the tooth, the selection of the correct restoration material, and the use of proper mechanical and micro-mechanical retention features.

**Correct Answer:** C. Mechanical retention.