Consider a framing protocol that uses a sequence of 011110 as a frame
delimiter. What should the bit stuffing rule be? If the data sequence
is 011011110 11101111110, what will be the transmitted bits?

Hi,
That depends of the protocol. Their are many ways a frame can see the
difference between a delimiter and normal data (that contains the same
data as the delimiter).

These are 2 often used methods:
- Put the size (n) of the frame before or after the delimiter. The
next n bits, bytes or any other unit are read without checking for
delimiter characters. The use of a delimiter character if even not
obligatory with this method.
- Make use of an 'escape sequence'. Before putting the input in the
frame, change the delimiter sequence in a different sequence. Eg. 000
is the escape sequence:
You declare 
00001 <- 011110
00010 <- 000

When you encode the data in your example (011011110 11101111110) you get:
01100001 11101111110

What happens when you want to encode the escape sequence itself (eg. 00011):
0001011

The use of the escape sequence can become complicated. In general it
is a better idea to always put the size of the frame in the packet.

Regards