Well, Let’s look at the options and see what we can deduce from them.
The length of the oleo is determined by the amount of oil and gas within it and the weight of the aircraft pressing down upon it. Any loss of oil or gas will reduce the length. Any excess of oil or gas will increase the length.
It is also worth noting that if the low gas charge pressure has been caused by a small gas leak, this leak will not be visually detectable. But an oil leak would be evident from the oil deposits in the area of the leak.
An internal (option A) leak would allow the oil and gas to mix, but would not change their total volume, so this would not change the oleo length. So option A is incorrect.
The length of the oleo is determined by the amount of fluid and gas which is held within it. So the tyre pressure (option D) is incorrect.
If the gas charge pressure is too high (option C) the oleo length will be greater than normal. So option C is incorrect.
If the gas charge pressure is too low (option B)the oleo will be shorter than normal, so this is the correct option.