In welding, proper weld penetration for butt joints is influenced by factors such as root gap and bevel angle. These parameters must be carefully selected to ensure effective fusion between the pieces being joined without causing excessive material wastage or defects.

Let's examine the options:

1. A root gap of 1 mm and a bevel angle of 30 degrees: These settings usually allow for shallow penetration, which may not be sufficient for thicker materials. Bevel angle influences the accessibility of the weld, and a 30-degree angle may lead to insufficient coverage or fusion.
2. A root gap of 3 mm and a bevel angle of 20 degrees: This combination provides a wider root gap for better penetration, but a 20-degree bevel angle might compromise weld quality by limiting the angle at which the weld pool can be effectively manipulated.
3. A root gap of 2 mm and a bevel angle of 60 degrees: This option presents a balance between penetration and fusion area. A 2 mm root gap is generally sufficient for adequate root penetration, and a 60-degree bevel angle provides excellent access and a wide surface area for the weld filler to cover. This helps in achieving deeper penetration even on thicker materials.
4. A root gap of 0.5 mm and a bevel angle of 45 degrees: This small root gap may restrict penetration and make it difficult to achieve a strong joint. A 45-degree bevel angle improves accessibility but may not compensate enough for the narrow gap.

Given the requirements for effective weld penetration and joint strength, the correct answer is: A root gap of 2 mm and a bevel angle of 60 degrees. This combination optimizes both access and penetration depth for typical welding applications.

Thus, the optimal choice for ensuring proper weld penetration in metal joining via a butt joint is Option C: A root gap of 2 mm and a bevel angle of 60 degrees.