In this sentence, we need to fill in the blank with the appropriate article. The correct article to use here is "a." Therefore, the complete sentence will read:

If a person has a legal problem, he will go and see a solicitor.

Here's a brief explanation of why we use "a":

• Indefinite Article: The article "a" is used before singular countable nouns when we are talking about something for the first time or when it is not specific. In this case, the person is not referring to a specific solicitor but rather any solicitor who can help with their legal problem.
• Countable Noun: "Solicitor" is a countable noun, meaning it can be counted (one solicitor, two solicitors, etc.). Therefore, it requires an article when used in the singular form.

In summary, when you refer to a profession or a role in a general sense, you typically use the indefinite article "a" or "an." In this case, "a solicitor" is the correct choice.