What’s the capture filter for a DHCP option?

The order of option 53 in the frame, and with that the position, is unknown. As capture filters don't have any protocol intelligence, you can't define a capture filter for a certain DHCP option.

The best thing you can do: Capture all DHCP/BOOTP frames and later use a display filter in Wireshark or tshark to filter only those frames with option 53.

Wireshark display filter

bootp.option.type == 53

Alternatively, you can use tshark with a display filter while you are capturing. Downside: you can't write a capture file (-w not supported with display filters). But you can print whatever fields you may need.

tshark -ni eth0 -Y "bootp.option.type == 53" -T fields -e frame.number -e frame.time -e ip.src -e ip.dst -e bootp.option.type -e bootp.ip.client -e xxxx

Replace xxxx with whatever bootp protocol field you may need.

http://www.wireshark.org/docs/dfref/b/bootp.html

Regards
Kurt