Given information:
Density = 1.984 g/cm^3
Length of the edge of the unit cell = 630 pm

To find the molar mass, we need to follow the following steps:

Step 1: Convert the length of the edge of the unit cell from picometers (pm) to centimeters (cm).
Step 2: Calculate the volume of the unit cell.
Step 3: Calculate the number of atoms in the unit cell.
Step 4: Calculate the mass of the unit cell.
Step 5: Calculate the molar mass.

Now let's go step by step:

Step 1: Convert the length of the edge of the unit cell from picometers (pm) to centimeters (cm).
1 pm = 1 × 10^(-10) cm
Length of the edge of the unit cell in cm = 630 pm × 1 × 10^(-10) cm/pm
Length of the edge of the unit cell in cm = 6.3 × 10^(-8) cm

Step 2: Calculate the volume of the unit cell.
For a face-centered cubic (FCC) crystal, the volume of the unit cell can be calculated using the formula:
Volume = (Length of the edge of the unit cell)^3
Volume = (6.3 × 10^(-8) cm)^3
Volume = 2.00 × 10^(-22) cm^3

Step 3: Calculate the number of atoms in the unit cell.
In a face-centered cubic (FCC) crystal, there are 4 atoms per unit cell.

Step 4: Calculate the mass of the unit cell.
Mass of the unit cell = Density × Volume
Mass of the unit cell = 1.984 g/cm^3 × 2.00 × 10^(-22) cm^3
Mass of the unit cell = 3.968 × 10^(-22) g

Step 5: Calculate the molar mass.
Molar mass = Mass of the unit cell / Number of atoms in the unit cell
Molar mass = 3.968 × 10^(-22) g / 4
Molar mass = 9.92 × 10^(-23) g

To convert the molar mass to g/mol, we need to multiply it by Avogadro's number (6.022 × 10^23 mol^(-1)).
Molar mass in g/mol = (9.92 × 10^(-23) g) × (6.022 × 10^23 mol^(-1))
Molar mass in g/mol = 59.6 g/mol

Therefore, the molar mass of the substance is 59.6 g/mol, which is closest to option D (74.70 g/mol).