The correct answer is option (c) frog.

Explanation:
During the construction of a brick building using modular bricks, the impression made on one face of the brick is called the frog. The frog is a shallow depression or indentation that is usually present on one or both bed faces (longitudinal faces) of a brick. It is typically located at the center of the bed face.

The purpose of the frog is to reduce the weight of the brick and also to provide a better grip or contact area for the mortar. The presence of the frog helps to improve the bond between the brick and the mortar, increasing the overall strength and stability of the structure.

The size and shape of the frog can vary depending on the type and size of the brick. It can be rectangular, square, circular, or triangular in shape. The depth of the frog is usually around 10-20 mm, but it can vary based on the brick manufacturer's specifications.

The frog is an important feature in brick construction as it allows for the proper placement of mortar. When laying bricks, the mortar is applied to the bed face of the brick, and the frog helps to accommodate the mortar and ensure proper bonding between the bricks. It also helps in controlling the amount of mortar used and prevents excessive mortar from squeezing out during the construction process.

The presence of the frog also helps in identifying the correct orientation of the brick. The frog should always face upwards during construction to ensure proper alignment and strength in the finished structure.

In conclusion, the impression made on one face of a brick is called the frog. It is a shallow depression or indentation that helps reduce the weight of the brick, improve mortar bonding, and ensure proper orientation during construction.