The answer to this question is very obvious when you look at the beginning of the story, which starts:
And when they came nigh to Jerusalem, unto Bethphage and Bethany, at the mount of Olives, he sendeth forth two of his disciples,
-- Mark 11:1 (KJV)
Notice here, Bethphage and Bethany are "at the mount of Olives". So we see at this point in the scripture, Jesus and his disciples were close to the mount of Olives.
Any one who has ever gone to an area where there is a mountain knows that you can actually see the Mountain visible from miles around.
Now if we go down in the text to Mark 11:12-14 we read the following
12 And on the morrow, when they were come from Bethany, he was hungry:
13 And seeing a fig tree afar off having leaves, he came, if haply he might find any thing thereon: and when he came to it, he found nothing but leaves; for the time of figs was not [yet].
14 And Jesus answered and said unto it, No man eat fruit of thee hereafter for ever. And his disciples heard [it]
Then notice that verse 15 begins by saying
15 And they come to Jerusalem: and Jesus went into the temple, and began
Matter of fact if you read Verse 1, verse 11 & 12, verse 15 and verse 27
You begin to notice that From verse 1 of Mark 11 through to the end of this chapter, Jesus journeys through this narrative was only from Bethany/Bethphage to Jerusalem severally.
Researching further, we find that Bethany is located about 1.5 miles (2.4 km) to the east of Jerusalem on the south-eastern slope of the Mount of Olives(Wikipedia). We also find the elevation of the Mount of Olives to be 826 m (2,710 ft) according to Wikipedia.
So if the fig tree that Peter saw was withered which Jesus spoke to earlier was located somewhere in between Bethany/Bethphage(where the mount of Olives is) and Jerusalem . Then no doubt the mount of Olives at its height of 826 m (2,710 ft) must have been pretty visible from where the withered tree was. And we can only conclude based on the facts from the text that the mountain Jesus spoke of must have been the mount of Olives.
Notice also that Jesus did not say 'a mountain', but he said 'this mountain'. So he was refering to a particular mountain which all evidence shows is the mount of Olives.