Option 1: Salt rock is not a mechanically formed sedimentary rock. It is a chemically formed sedimentary rock, which means it forms from the precipitation of minerals from a solution.

Option 2: Limestone is not a mechanically formed sedimentary rock. It is formed from the accumulation of organic remains, such as shells or coral, or from the precipitation of calcium carbonate.

Option 3: Sandstone is the correct answer. It is a mechanically formed sedimentary rock that is composed of grains of sand that have been lithified or cemented together. These grains could have originated from weathering and erosion of existing rocks.

Option 4: Gypsum is not a mechanically formed sedimentary rock. It is a chemically formed sedimentary rock, which forms when evaporation of water from an ancient sea or salt lake leaves behind the mineral gypsum.

Therefore, the correct answer is option 3, sandstone. It is a mechanically formed sedimentary rock composed of sand grains that have been lithified together.