Refer to figures. 
Firstly, find the CAS, using your Flight/Nav computer:

• In the airspeed correction window, align the OAT at FL150 (M15) with 15 000 ft pressure altitude.
• Below TAS 176 kt on the outer scale, read CAS 140 kt on the inner scale.

Using again your Flight/Nav computer, find the TAS at FL210:

• In the airspeed correction window, align the OAT at FL210 (M25) with 21 000 ft pressure altitude.
• Above CAS 140 kt on the inner scale, read TAS 196 kt on the outer scale.

To calculate the revised ETA at WPT C, the Groundspeed at FL210 is required, through your Flight/Nav computer:

• Place the centre dot over TAS 196 kt.
• Rotate the disk until wind direction 190° is under the True index.
• Mark wind velocity 5 kt below the centre dot.
• Rotate the disc until the track 335° is under the True index.
• The mark shows a Groundspeed of 200 kt.

Thus, the sector from WPT B to WPT C will be covered in: Sector Time = Sector distance / GS = 61 NM / 200 kt = 0.3 hr or 18 min.

Therefore, the revised ETA at WPT C will be: 12:12 (ATA at WPT B) + 18 min = 12:30. Closest option: 12:28.