Ten.

It could be a kind of ritardando, precisely held....

In my opinion, it is just a precaution by Haydn to indicate that those notes should be held until the end. If it were an agogic indication, he would have written it on all the staves... how is it that only 3 instruments retard and the others do not?

Marzapane wrote:

While studying Haydn's Symphony No. 99, at measure 6 of the first movement, I see 'ten.' indicated for the strings (vc, cb and Vn II).

Why?

That indication is also on the first chord.

The indication 'ten.' stands for «tenuto», meaning: the sound held for its entire duration.

Cf. for example, the last measures of the introductory Adagio of Beethoven's First Symphony.

...ok, I was mistaken