ACSR Conductor:
ACSR (Aluminum Conductor Steel Reinforced) is a type of overhead conductor widely used in electrical power transmission and distribution lines. It consists of a central steel core surrounded by one or more layers of aluminum strands.

Number of Strands in ACSR Conductor:
The number of strands in an ACSR conductor depends on the number of layers present in its construction. Each layer adds a certain number of strands to the conductor. Let's analyze the given options to determine the correct number of strands for 3 layers of ACSR conductor.

Option A: 1 Strand
If there is only 1 strand, it means there is no additional layer of aluminum strands. This contradicts the information that there are 3 layers in the ACSR conductor. Therefore, option A is incorrect.

Option B: 7 Strands
If there are 7 strands, it means there are 6 additional strands apart from the central steel core. This implies that there are 2 layers of aluminum strands, as each layer adds 3 strands. However, the given information states that there are 3 layers, which means there should be more than 7 strands. Therefore, option B is incorrect.

Option C: 19 Strands
If there are 19 strands, it means there are 18 additional strands apart from the central steel core. This implies that there are 6 layers of aluminum strands, as each layer adds 3 strands. However, the given information states that there are 3 layers, which means there should be fewer than 19 strands. Therefore, option C is incorrect.

Option D: 37 Strands
If there are 37 strands, it means there are 36 additional strands apart from the central steel core. This implies that there are 12 layers of aluminum strands, as each layer adds 3 strands. Therefore, option D is the correct answer.

Conclusion:
For 3 layers of ACSR conductor, the correct number of strands is 37 (option D).