Translation:Glass is cheaper than aluminum.
In the previous sentence the definite article before "silicon" was not accepted but here as in other exercises a material plus definite article was given as an alternative. That said in English the definite article is not used unless it is in relation to a specific example of the metal.
We are accepting these both with and without the definite article because without context the meaning can be interpreted in various ways. For example here if describing the costs of glass and aluminum in general we wouldn't use an article. But if we said: "Which door will you buy." we might use an article.
We have made up our mind but if we don't see what you have we don't know what the issue is.
Here are the sentences we have.
Το γυαλί είναι πιο φθηνό από το αλουμίνιο.
Το γυαλί είναι πιο φτηνό από το αλουμίνιο.
Το γυαλί είναι φτηνότερο από το αλουμίνιο. Τ Tο γυαλί είναι φθηνότερο από το αλουμίνιο.
These should lead you to realize that the problem is not with a "τ" ή "θ".
Now if you would do the same and show us the full sentence by using the Report option then we would know where the problem is.