What will allow you to accomplish this?

I think the answer is B too.
The EXP bits (Experimental Bits) in the MPLS label are used to classify a packet for COS.
By default, egress router advertises Label 3 (implicit null) to the penultimate router which makes the penultimate router to strip the MPLS label and send pure IP packet.

So if you want to classify packets using EXP bits, you have to send MPLS label to the Egress router hence have to ask the egress router to advertise explicit null (label 0) to the penultimate router so that penultimate router can send packet with MPLS label which has the EXP bits.

check out below sources

http://www.juniper.net/techpubs/en_US/junos13.2/topics/usage-guidelines/cos-applying-mpls-exp-classifiers-for-explicit-null-labels.html

Applying MPLS EXP Classifiers for Explicit-Null Labels

When you configure MPLS explicit-null labels, label 0 is advertised to the egress router of an LSP. When label 0 is advertised, the egress router (instead of the penultimate router) removes the label. Ultimate-hop popping ensures that any packets traversing an MPLS network include a label.

On M320 and T Series routers, when you configure MPLS explicit-null labels with an MPLS EXP classifier, the MPLS EXP classifier can be different from an IPv4 or IPv6 classifier configured on the same logical interface. In other words, you can apply separate classifiers for MPLS EXP, IPv4, and IPv6 packets per logical interface. To combine an EXP classifier with a distinct IPv6 classifier, the PIC must be mounted on an Enhanced FPC

http://www.juniper.net/techpubs/en_US/junos13.2/topics/reference/configuration-statement/explicit-null-edit-protocols-mpls.html

Syntax
explicit-null;

Hierarchy Level
[edit logical-systems logical-system-name protocols mpls],
[edit protocols mpls]

http://www.juniper.net/techpubs/en_US/junos12.2/topics/usage-guidelines/routing-advertising-explicit-null-labels-to-bgp-peers.html

Advertising Explicit Null Labels to BGP Peers

For the IPv4 (inet) family only, BGP peers in a routing group can send an explicit NULL label for a set of connected routes (direct and loopback routes) for the inet labeled-unicast and inet6 labeled-unicast NLRI.
By default, peers advertise label 3 (implicit NULL). If the explicit-null statement is enabled, peers advertise label 0 (explicit NULL).
The explicit NULL labels ensures that labels are always present on packets traversing an MPLS network.
If the implicit NULL label is used. the penultimate hop router removes the label and sends the packet as a plain IP packet to the egress router. This might cause issues in queuing the packet properly on the penultimate hop router if the penultimate hop is another vendor’s router. Some other vendors queue packets based on the CoS bits in the outgoing label rather than the incoming label.

To advertise an explicit null label, include the following statements in the configuration:

family inet {
labeled-unicast {
aggregate-label {
community community-name:
}
explicit-null {
connected-only;
}
}
}