Simple multiplication quiz
First of all, you have the following construct:

if yn == "n":
    exit()
else:
    lots
    of
    code


But if yn equals "n", the exit() will have been executed, and you won't reach any other lines anyway. So you can just write

if yn == "n":
    exit()

lots
of
code


This is like an 'early return' (except not in a function, but in the main body of code).

Second, you have

x =int( random.choice(range(1,100)))
...
a = str(x)
...
print "What is " + a + " x " + b + "?"


The spacing on the line with x = is off. It should be

x = int(random.choice(range(1, 100))


That is: no spaces next to ( and ). A space on each side of the =, and a space after the ,.

Furthermore, you should really use string formatting, allowing you to write

print "What is {} + {}?".format(x, y)


This way you don't need intermediate variables a and b.

I assume ua means user_answer here:

print "What is " + a  + " x " + b + "?"
ua = int(raw_input("What is the answer? "))


Why not just write

problem = "What is {} x {}? ".format(x, y)
user_answer = int(raw_input(problem))


You're basically asking a question, and then "What is the answer to the question?". Better to just ask the question alone.

Instead of writing xy, you could write solution or answer.

Another thing of note: the score (I assume that's what you mean with s) gets reset to 0 each time you get a new problem statement, but the score (t) is only printed at the end of the rounds.

That is, you have (pseudo-code)

while play_again:
    ...
    s = 0
    answered = ...
    while answered != solution:
        answered = ...
        s -= 1  # You had s = s - 1, that is equivalent to s -= 1
    s += 1
    t = str(s)
print "Your score is " + t


While instead I think you mean 

s = 0
while play_again:
    ...
    answered = ...
    while answered != solution:
        answered = ...
        s -= 1  # You had s = s - 1, that is equivalent to s -= 1
    s += 1
t = str(s)
print "Your score is " + t


Note that the last two lines can be replaced with

print "Your score is {}".format(s)


(Also note: if the user inputs something different from y or n on the first go, you get a NameError: t is referenced before it is assigned because you never enter the while loop).