How many paths to a LUN can be simultaneously used when Round Robin multi-pathing is enabled?

A.
Round Robin will use only one path to send I/O

B.
Round Robin can use up to 4 paths simultaneously to send I/O

C.
Round Robin can be set to use additional paths simultaneously for every 1000 blocks of I/O transmitted

D.
Round Robin can use up to 2 paths simultaneously to send I/O

Explanation:
Fibre Channel SAN Configuration Guide ESX ESXi vCenter Server, page 25.

Round Robin (RR) Uses a path selection algorithm that rotates through all available paths enabling
load balancing across the paths.

Round-Robin Load Balancing
When one path from the ESX Server host to the SAN becomes unavailable, the host switches to another path. ESX Server hosts can also use multipathing for load balancing.

When to switch Specify that the ESX Server host should attempt a path switch after a specified number of I/O blocks have been issued on a path or after a specified number of read or write commands have been issued on a path. If another path exists that meets the specified path policy for the target, the active path to the target is switched to the new path.

Since the active path is switched to a new path, it can be assumed that only a single path is ever used at any given time, (A above).