The correct answer is option A. 

Explanation: 
When the three glass tubes packed with clay, sand, and loamy soils are kept in a beaker of water for one week, the water will move through the soil and fill the empty spaces in the tubes. The level of water in the tubes will depend on the porosity of each soil type. 

Clay soil has smaller pores and is more tightly packed, which means it has less space for water to move through. This results in the lowest level of water in the tube with clay soil. 

Sand has larger pores and is less tightly packed, which means it has more space for water to move through. This results in a higher level of water in the tube with sandy soil. 

Loamy soil has a mixture of different particle sizes and is moderately packed, which means it has an intermediate level of porosity. Therefore, the water level in the tube with loamy soil will be somewhere between the levels in the tubes with clay and sand. 

Hence, the correct option is A - lowest in the tube with clay.