BPDU Frame EtherType – Spanning Tree Protocol on Switches

The vSRX doesn't support switching features, so the answer is:

(b) the SRX doesn't support spanning-tree features.

vSRX is designed to be a virtual firewall, therefore it doesn't need "switching". It's only the physical SRX devices, that support switching. Take a look at this product datasheet from Juniper, it lists all of the available features on that platform.

The short answer is that MSTP actually uses a special instance in addition to the user defined ones to handle all MSTP control traffic, called the Internal Spanning Tree instance (IST0).

From: http://blog.ine.com/2010/02/22/understanding-mstp/

The IEEE 802.1s implementation does not send BDPUs for every active STP instance separately, nor does it encapsulate VLAN numbers list configuration messages. Instead, a special STP instance number 0 called Internal Spanning Tree (IST aka MSTI0, Multiple Spanning Tree Instance 0) is designated to carry all STP-related information. The BPDUs for IST contain all standard RSTP-style information for the IST itself, as well as carry additional informational fields. Among those fields are configuration name, revision number and a hash value calculated over VLANs to MSTI mapping table contents. Using just this condensed information switches may detect mis-configuration in VLAN mappings by comparing the hash value received from the peer with the local value.

You should definitely check out the rest of that white paper, it's a great resource.