Answer: a
In this context, express means explicitly stated. Expressed just means communicated in some way. And, as Warren Clements explained, if you’ve got permission, you’ve got expressed permission (really, how else would you know?):

“…expressed written permission is redundant, since if the permission is written it is by definition expressed. To express, from exprimere by way of the medieval Latin expressare (also to press out), means to put into words. Unlike express permission, expressed permission may be merely implied, which is why Major League Baseball, in one section of a long legal disclaimer, says it makes no warranties of any kind, “either express or implied.” Expressed can even mask a lie: The visitors’ expressed intention was to check the house’s wiring, but they were really casing the joint.”

Although you might think that there’s not much difference between express and expressed and there’s no point in being persnickety about it, we need express to continue to mean what it means.

Express is used in law, for example, to differentiate an explicit contract from an implicit contract. Lucas Gonze explains implicit contracts here:

“An implicit contract is one where the terms are NOT
expressed, neither orally nor on paper. You walk
into a restaurant and sit down. The waitress asks,
“What would you like?” You say, “A slice of apple
pie sounds good.” After you eat, she brings a bill.
What? A bill?!? You thought she was just being
kind. Never did you *explicitly* agree to pay for the
pie. Well .. the court isn’t going to buy that. You
made an implicit contract when you went through
the socially conventional mechanisms for eating
at a restaurant, thereby accepting the socially
conventional, implied contract for doing so.

The world would be a messy place if it weren’t
for implicit contracts. Cafeterias, restaurants,
and gas stations would be far less convenient and
carry far more friction if we had to make explicit
contracts before engaging in business.”

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Answer

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Answer: a
In this context, express means both explicitly stated and for this use only.