Deze paprika is zo vers dat de hele salade lekker ruikt.

Because paprika is a de-word in Dutch: de paprika.

For demonstratives:

• deze goes with de-words
• dit goes with het-words

So:

• deze paprika = this bell pepper
• dit boek = this book

Even though paprika ends in -a, that does not make it a het-word.

Here it means the vegetable: a bell pepper / sweet pepper.

In Dutch, paprika can mean:

• the vegetable
• the spice paprika powder

The context makes it clear. Since the sentence says Deze paprika is zo vers..., it is talking about a fresh vegetable.

Zo ... dat means so ... that.

In this sentence:

• zo vers = so fresh
• dat de hele salade lekker ruikt = that the whole salad smells nice

So the structure is:

• zo + adjective + dat + clause
• so + adjective + that + clause

Examples:

• Hij is zo moe dat hij meteen slaapt.
  He is so tired that he falls asleep immediately.
• Het is zo koud dat ik binnen blijf.
  It is so cold that I stay inside.

Because dat introduces a subordinate clause, and in Dutch subordinate clauses the conjugated verb usually goes to the end.

Main clause:

• Deze paprika is zo vers

Subordinate clause:

• dat de hele salade lekker ruikt

Compare with English:

• Dutch: ... dat de hele salade lekker ruikt
• English: ... that the whole salad smells nice

This verb-final position is one of the most important Dutch word-order patterns to learn.

Because in this sentence the noun phrase is definite: the whole salad.

So Dutch uses:

• de hele salade

Not:

• hele salade in this context

A few useful comparisons:

• de hele salade = the whole salad
• een hele salade = a whole salad
• hele salades = whole salads

So hele usually appears together with an article or determiner when it means whole / entire.

Yes. In Dutch, lekker is used much more broadly than English tasty.

It can describe:

• food that tastes good
• something that smells good
• pleasant weather
• a comfortable feeling
• something enjoyable in general

So:

• lekker ruiken = to smell nice / smell good
• lekker eten = to eat well / enjoy food
• lekker warm = pleasantly warm

In natural English, you would usually translate lekker ruikt here as smells nice or smells good, not smells tasty.

Because vers is used predicatively here, after the verb is.

In Dutch:

• predicative adjective: no -e ending
  
  • De paprika is vers.
• attributive adjective: often takes -e
  
  • de verse paprika

So in your sentence:

• Deze paprika is zo vers...
  not zo verse

This is a very common difference in Dutch adjectives.

Because the subject of the subordinate clause is de hele salade, which is singular.

Breakdown:

• subject: de hele salade
• verb: ruikt

So:

• de salade ruikt = the salad smells
• de salades ruiken = the salads smell

Even though a salad contains many ingredients, the grammatical subject is still one singular noun: salade.

They are related, but they are used differently.

• heel can mean very
  
  • heel vers = very fresh
• heel / hele can also mean whole / entire
  
  • de hele salade = the whole salad

In your sentence:

• zo vers means so fresh
• de hele salade means the whole salad

So hele here does not mean very. It means entire.

Not if you want the same so ... that structure.

• zo vers dat ... = so fresh that ...
• erg vers = very fresh
• heel vers = very fresh

If you say:

• Deze paprika is erg vers.
  that just means This bell pepper is very fresh.

But:

• Deze paprika is zo vers dat de hele salade lekker ruikt.
  means This bell pepper is so fresh that the whole salad smells nice.

So zo is the correct choice when it is followed by dat to express a result.

It is grammatically natural, but semantically it is a bit expressive or dramatic.

The idea is:

• the pepper is extremely fresh
• its freshness affects the smell of the entire salad

Dutch often uses this kind of sentence to emphasize quality. So it sounds fine, but it is not just a plain neutral description. It has a slightly vivid, promotional, or enthusiastic feel.