[excellency]

which one completes this sentence?
that tree is 15 meters............
a)long b)tall
thank you.

[susan53]

Possibly tall, but also high..

Long referring to distance (as opposed to time) is always horizontal - eg The garden is 120 metres long and 30 metres wide.

Tall is used to describe height for people - John is 5ft tall - and can be used for certain objects - eg tall buildings and also tall trees :

A picturesque lake surrounded by tall trees.

But high is also used :

A woodland road, flanked by high trees in the foreground,

So you could use either in the sentence you quote.