[susan53]

d) A gang of youths were loitering/lingering outside the cinema.

These two aren't ways of walking at all but of "remaining" somewhere.

Linger : This means that something/someone stays somewhere longer than is necessary or expected.
They lingered over coffee and missed the last bus
Elisabeth did not enter the museum at once, but lingered in the courtyard.
...evenings are delightful and everyone wants to linger under the stars.
..his eyes lingered on Nick's face, then moved back to Elaine

Interestingly it is less often used with reference to people than to abstract ideas and with other non-human referents :
..a brewing fight over bilingual education and lingering animosity toward Gov. Pete Wilson.
The result removed any lingering doubt on this issue..
The cold lingered, making sleep difficult that night,..
The memory lingered on, though, in the minds of many...
It's frequently used to describe a long drawn out death, whether real or metaphorical :
...any white wine will die a lingering death when it is allowed to warm

Notice that there is nothing negative about it though. If your example had been The boys lingered outside the cinema for a while, talking quietly it would have been fine. But the expression "gang of youths" is loaded with negative assumptions - it suggests the boys were there for some disruptive purpose. So here, lingered sounds strange.

loiter does often have a negative connotation. Strictly the meaning is to remain somewhere without a specific purpose but it is frequently associated with crime, and may describe an actual criminal offence (loitering with intent). These are examples from press reports :
A 65-year-old Aberdeen councillor and former Holyrood candidate is fined after admitting loitering in Aberdeen's red light district.
Mugger loitered near shop in heavy rain before assault
Charges are filed against a repeat sex offender after he is found loitering at a Norman park watching a youth ...
Even when there is no real crime involved it is seen as "hanging around" that may disturb other people :
A mall installs an anti-loitering device. The device makes a noise that discourages loitering by teens or anybody else who may be loitering ...
Because of the negative connotations of the phrase a gang of youths therefore, loiter would be much more likely to be used in this context than linger.